Source file src/go/parser/parser.go

     1  // Copyright 2009 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package parser implements a parser for Go source files. Input may be
     6  // provided in a variety of forms (see the various Parse* functions); the
     7  // output is an abstract syntax tree (AST) representing the Go source. The
     8  // parser is invoked through one of the Parse* functions.
     9  //
    10  // The parser accepts a larger language than is syntactically permitted by
    11  // the Go spec, for simplicity, and for improved robustness in the presence
    12  // of syntax errors. For instance, in method declarations, the receiver is
    13  // treated like an ordinary parameter list and thus may contain multiple
    14  // entries where the spec permits exactly one. Consequently, the corresponding
    15  // field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
    16  package parser
    17  
    18  import (
    19  	"fmt"
    20  	"go/ast"
    21  	"go/internal/typeparams"
    22  	"go/scanner"
    23  	"go/token"
    24  )
    25  
    26  // The parser structure holds the parser's internal state.
    27  type parser struct {
    28  	file    *token.File
    29  	errors  scanner.ErrorList
    30  	scanner scanner.Scanner
    31  
    32  	// Tracing/debugging
    33  	mode   Mode // parsing mode
    34  	trace  bool // == (mode&Trace != 0)
    35  	indent int  // indentation used for tracing output
    36  
    37  	// Comments
    38  	comments    []*ast.CommentGroup
    39  	leadComment *ast.CommentGroup // last lead comment
    40  	lineComment *ast.CommentGroup // last line comment
    41  
    42  	// Next token
    43  	pos token.Pos   // token position
    44  	tok token.Token // one token look-ahead
    45  	lit string      // token literal
    46  
    47  	// Error recovery
    48  	// (used to limit the number of calls to parser.advance
    49  	// w/o making scanning progress - avoids potential endless
    50  	// loops across multiple parser functions during error recovery)
    51  	syncPos token.Pos // last synchronization position
    52  	syncCnt int       // number of parser.advance calls without progress
    53  
    54  	// Non-syntactic parser control
    55  	exprLev int  // < 0: in control clause, >= 0: in expression
    56  	inRhs   bool // if set, the parser is parsing a rhs expression
    57  
    58  	imports []*ast.ImportSpec // list of imports
    59  
    60  	// nestLev is used to track and limit the recursion depth
    61  	// during parsing.
    62  	nestLev int
    63  }
    64  
    65  func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
    66  	p.file = fset.AddFile(filename, -1, len(src))
    67  	var m scanner.Mode
    68  	if mode&ParseComments != 0 {
    69  		m = scanner.ScanComments
    70  	}
    71  	eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
    72  	p.scanner.Init(p.file, src, eh, m)
    73  
    74  	p.mode = mode
    75  	p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)
    76  	p.next()
    77  }
    78  
    79  // ----------------------------------------------------------------------------
    80  // Parsing support
    81  
    82  func (p *parser) printTrace(a ...any) {
    83  	const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
    84  	const n = len(dots)
    85  	pos := p.file.Position(p.pos)
    86  	fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
    87  	i := 2 * p.indent
    88  	for i > n {
    89  		fmt.Print(dots)
    90  		i -= n
    91  	}
    92  	// i <= n
    93  	fmt.Print(dots[0:i])
    94  	fmt.Println(a...)
    95  }
    96  
    97  func trace(p *parser, msg string) *parser {
    98  	p.printTrace(msg, "(")
    99  	p.indent++
   100  	return p
   101  }
   102  
   103  // Usage pattern: defer un(trace(p, "..."))
   104  func un(p *parser) {
   105  	p.indent--
   106  	p.printTrace(")")
   107  }
   108  
   109  // maxNestLev is the deepest we're willing to recurse during parsing
   110  const maxNestLev int = 1e5
   111  
   112  func incNestLev(p *parser) *parser {
   113  	p.nestLev++
   114  	if p.nestLev > maxNestLev {
   115  		p.error(p.pos, "exceeded max nesting depth")
   116  		panic(bailout{})
   117  	}
   118  	return p
   119  }
   120  
   121  // decNestLev is used to track nesting depth during parsing to prevent stack exhaustion.
   122  // It is used along with incNestLev in a similar fashion to how un and trace are used.
   123  func decNestLev(p *parser) {
   124  	p.nestLev--
   125  }
   126  
   127  // Advance to the next token.
   128  func (p *parser) next0() {
   129  	// Because of one-token look-ahead, print the previous token
   130  	// when tracing as it provides a more readable output. The
   131  	// very first token (!p.pos.IsValid()) is not initialized
   132  	// (it is token.ILLEGAL), so don't print it.
   133  	if p.trace && p.pos.IsValid() {
   134  		s := p.tok.String()
   135  		switch {
   136  		case p.tok.IsLiteral():
   137  			p.printTrace(s, p.lit)
   138  		case p.tok.IsOperator(), p.tok.IsKeyword():
   139  			p.printTrace("\"" + s + "\"")
   140  		default:
   141  			p.printTrace(s)
   142  		}
   143  	}
   144  
   145  	p.pos, p.tok, p.lit = p.scanner.Scan()
   146  }
   147  
   148  // Consume a comment and return it and the line on which it ends.
   149  func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
   150  	// /*-style comments may end on a different line than where they start.
   151  	// Scan the comment for '\n' chars and adjust endline accordingly.
   152  	endline = p.file.Line(p.pos)
   153  	if p.lit[1] == '*' {
   154  		// don't use range here - no need to decode Unicode code points
   155  		for i := 0; i < len(p.lit); i++ {
   156  			if p.lit[i] == '\n' {
   157  				endline++
   158  			}
   159  		}
   160  	}
   161  
   162  	comment = &ast.Comment{Slash: p.pos, Text: p.lit}
   163  	p.next0()
   164  
   165  	return
   166  }
   167  
   168  // Consume a group of adjacent comments, add it to the parser's
   169  // comments list, and return it together with the line at which
   170  // the last comment in the group ends. A non-comment token or n
   171  // empty lines terminate a comment group.
   172  func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
   173  	var list []*ast.Comment
   174  	endline = p.file.Line(p.pos)
   175  	for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
   176  		var comment *ast.Comment
   177  		comment, endline = p.consumeComment()
   178  		list = append(list, comment)
   179  	}
   180  
   181  	// add comment group to the comments list
   182  	comments = &ast.CommentGroup{List: list}
   183  	p.comments = append(p.comments, comments)
   184  
   185  	return
   186  }
   187  
   188  // Advance to the next non-comment token. In the process, collect
   189  // any comment groups encountered, and remember the last lead and
   190  // line comments.
   191  //
   192  // A lead comment is a comment group that starts and ends in a
   193  // line without any other tokens and that is followed by a non-comment
   194  // token on the line immediately after the comment group.
   195  //
   196  // A line comment is a comment group that follows a non-comment
   197  // token on the same line, and that has no tokens after it on the line
   198  // where it ends.
   199  //
   200  // Lead and line comments may be considered documentation that is
   201  // stored in the AST.
   202  func (p *parser) next() {
   203  	p.leadComment = nil
   204  	p.lineComment = nil
   205  	prev := p.pos
   206  	p.next0()
   207  
   208  	if p.tok == token.COMMENT {
   209  		var comment *ast.CommentGroup
   210  		var endline int
   211  
   212  		if p.file.Line(p.pos) == p.file.Line(prev) {
   213  			// The comment is on same line as the previous token; it
   214  			// cannot be a lead comment but may be a line comment.
   215  			comment, endline = p.consumeCommentGroup(0)
   216  			if p.file.Line(p.pos) != endline || p.tok == token.SEMICOLON || p.tok == token.EOF {
   217  				// The next token is on a different line, thus
   218  				// the last comment group is a line comment.
   219  				p.lineComment = comment
   220  			}
   221  		}
   222  
   223  		// consume successor comments, if any
   224  		endline = -1
   225  		for p.tok == token.COMMENT {
   226  			comment, endline = p.consumeCommentGroup(1)
   227  		}
   228  
   229  		if endline+1 == p.file.Line(p.pos) {
   230  			// The next token is following on the line immediately after the
   231  			// comment group, thus the last comment group is a lead comment.
   232  			p.leadComment = comment
   233  		}
   234  	}
   235  }
   236  
   237  // A bailout panic is raised to indicate early termination. pos and msg are
   238  // only populated when bailing out of object resolution.
   239  type bailout struct {
   240  	pos token.Pos
   241  	msg string
   242  }
   243  
   244  func (p *parser) error(pos token.Pos, msg string) {
   245  	if p.trace {
   246  		defer un(trace(p, "error: "+msg))
   247  	}
   248  
   249  	epos := p.file.Position(pos)
   250  
   251  	// If AllErrors is not set, discard errors reported on the same line
   252  	// as the last recorded error and stop parsing if there are more than
   253  	// 10 errors.
   254  	if p.mode&AllErrors == 0 {
   255  		n := len(p.errors)
   256  		if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
   257  			return // discard - likely a spurious error
   258  		}
   259  		if n > 10 {
   260  			panic(bailout{})
   261  		}
   262  	}
   263  
   264  	p.errors.Add(epos, msg)
   265  }
   266  
   267  func (p *parser) errorExpected(pos token.Pos, msg string) {
   268  	msg = "expected " + msg
   269  	if pos == p.pos {
   270  		// the error happened at the current position;
   271  		// make the error message more specific
   272  		switch {
   273  		case p.tok == token.SEMICOLON && p.lit == "\n":
   274  			msg += ", found newline"
   275  		case p.tok.IsLiteral():
   276  			// print 123 rather than 'INT', etc.
   277  			msg += ", found " + p.lit
   278  		default:
   279  			msg += ", found '" + p.tok.String() + "'"
   280  		}
   281  	}
   282  	p.error(pos, msg)
   283  }
   284  
   285  func (p *parser) expect(tok token.Token) token.Pos {
   286  	pos := p.pos
   287  	if p.tok != tok {
   288  		p.errorExpected(pos, "'"+tok.String()+"'")
   289  	}
   290  	p.next() // make progress
   291  	return pos
   292  }
   293  
   294  // expect2 is like expect, but it returns an invalid position
   295  // if the expected token is not found.
   296  func (p *parser) expect2(tok token.Token) (pos token.Pos) {
   297  	if p.tok == tok {
   298  		pos = p.pos
   299  	} else {
   300  		p.errorExpected(p.pos, "'"+tok.String()+"'")
   301  	}
   302  	p.next() // make progress
   303  	return
   304  }
   305  
   306  // expectClosing is like expect but provides a better error message
   307  // for the common case of a missing comma before a newline.
   308  func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
   309  	if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
   310  		p.error(p.pos, "missing ',' before newline in "+context)
   311  		p.next()
   312  	}
   313  	return p.expect(tok)
   314  }
   315  
   316  // expectSemi consumes a semicolon and returns the applicable line comment.
   317  func (p *parser) expectSemi() (comment *ast.CommentGroup) {
   318  	// semicolon is optional before a closing ')' or '}'
   319  	if p.tok != token.RPAREN && p.tok != token.RBRACE {
   320  		switch p.tok {
   321  		case token.COMMA:
   322  			// permit a ',' instead of a ';' but complain
   323  			p.errorExpected(p.pos, "';'")
   324  			fallthrough
   325  		case token.SEMICOLON:
   326  			if p.lit == ";" {
   327  				// explicit semicolon
   328  				p.next()
   329  				comment = p.lineComment // use following comments
   330  			} else {
   331  				// artificial semicolon
   332  				comment = p.lineComment // use preceding comments
   333  				p.next()
   334  			}
   335  			return comment
   336  		default:
   337  			p.errorExpected(p.pos, "';'")
   338  			p.advance(stmtStart)
   339  		}
   340  	}
   341  	return nil
   342  }
   343  
   344  func (p *parser) atComma(context string, follow token.Token) bool {
   345  	if p.tok == token.COMMA {
   346  		return true
   347  	}
   348  	if p.tok != follow {
   349  		msg := "missing ','"
   350  		if p.tok == token.SEMICOLON && p.lit == "\n" {
   351  			msg += " before newline"
   352  		}
   353  		p.error(p.pos, msg+" in "+context)
   354  		return true // "insert" comma and continue
   355  	}
   356  	return false
   357  }
   358  
   359  func assert(cond bool, msg string) {
   360  	if !cond {
   361  		panic("go/parser internal error: " + msg)
   362  	}
   363  }
   364  
   365  // advance consumes tokens until the current token p.tok
   366  // is in the 'to' set, or token.EOF. For error recovery.
   367  func (p *parser) advance(to map[token.Token]bool) {
   368  	for ; p.tok != token.EOF; p.next() {
   369  		if to[p.tok] {
   370  			// Return only if parser made some progress since last
   371  			// sync or if it has not reached 10 advance calls without
   372  			// progress. Otherwise consume at least one token to
   373  			// avoid an endless parser loop (it is possible that
   374  			// both parseOperand and parseStmt call advance and
   375  			// correctly do not advance, thus the need for the
   376  			// invocation limit p.syncCnt).
   377  			if p.pos == p.syncPos && p.syncCnt < 10 {
   378  				p.syncCnt++
   379  				return
   380  			}
   381  			if p.pos > p.syncPos {
   382  				p.syncPos = p.pos
   383  				p.syncCnt = 0
   384  				return
   385  			}
   386  			// Reaching here indicates a parser bug, likely an
   387  			// incorrect token list in this function, but it only
   388  			// leads to skipping of possibly correct code if a
   389  			// previous error is present, and thus is preferred
   390  			// over a non-terminating parse.
   391  		}
   392  	}
   393  }
   394  
   395  var stmtStart = map[token.Token]bool{
   396  	token.BREAK:       true,
   397  	token.CONST:       true,
   398  	token.CONTINUE:    true,
   399  	token.DEFER:       true,
   400  	token.FALLTHROUGH: true,
   401  	token.FOR:         true,
   402  	token.GO:          true,
   403  	token.GOTO:        true,
   404  	token.IF:          true,
   405  	token.RETURN:      true,
   406  	token.SELECT:      true,
   407  	token.SWITCH:      true,
   408  	token.TYPE:        true,
   409  	token.VAR:         true,
   410  }
   411  
   412  var declStart = map[token.Token]bool{
   413  	token.IMPORT: true,
   414  	token.CONST:  true,
   415  	token.TYPE:   true,
   416  	token.VAR:    true,
   417  }
   418  
   419  var exprEnd = map[token.Token]bool{
   420  	token.COMMA:     true,
   421  	token.COLON:     true,
   422  	token.SEMICOLON: true,
   423  	token.RPAREN:    true,
   424  	token.RBRACK:    true,
   425  	token.RBRACE:    true,
   426  }
   427  
   428  // safePos returns a valid file position for a given position: If pos
   429  // is valid to begin with, safePos returns pos. If pos is out-of-range,
   430  // safePos returns the EOF position.
   431  //
   432  // This is hack to work around "artificial" end positions in the AST which
   433  // are computed by adding 1 to (presumably valid) token positions. If the
   434  // token positions are invalid due to parse errors, the resulting end position
   435  // may be past the file's EOF position, which would lead to panics if used
   436  // later on.
   437  func (p *parser) safePos(pos token.Pos) (res token.Pos) {
   438  	defer func() {
   439  		if recover() != nil {
   440  			res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
   441  		}
   442  	}()
   443  	_ = p.file.Offset(pos) // trigger a panic if position is out-of-range
   444  	return pos
   445  }
   446  
   447  // ----------------------------------------------------------------------------
   448  // Identifiers
   449  
   450  func (p *parser) parseIdent() *ast.Ident {
   451  	pos := p.pos
   452  	name := "_"
   453  	if p.tok == token.IDENT {
   454  		name = p.lit
   455  		p.next()
   456  	} else {
   457  		p.expect(token.IDENT) // use expect() error handling
   458  	}
   459  	return &ast.Ident{NamePos: pos, Name: name}
   460  }
   461  
   462  func (p *parser) parseIdentList() (list []*ast.Ident) {
   463  	if p.trace {
   464  		defer un(trace(p, "IdentList"))
   465  	}
   466  
   467  	list = append(list, p.parseIdent())
   468  	for p.tok == token.COMMA {
   469  		p.next()
   470  		list = append(list, p.parseIdent())
   471  	}
   472  
   473  	return
   474  }
   475  
   476  // ----------------------------------------------------------------------------
   477  // Common productions
   478  
   479  // If lhs is set, result list elements which are identifiers are not resolved.
   480  func (p *parser) parseExprList() (list []ast.Expr) {
   481  	if p.trace {
   482  		defer un(trace(p, "ExpressionList"))
   483  	}
   484  
   485  	list = append(list, p.parseExpr())
   486  	for p.tok == token.COMMA {
   487  		p.next()
   488  		list = append(list, p.parseExpr())
   489  	}
   490  
   491  	return
   492  }
   493  
   494  func (p *parser) parseList(inRhs bool) []ast.Expr {
   495  	old := p.inRhs
   496  	p.inRhs = inRhs
   497  	list := p.parseExprList()
   498  	p.inRhs = old
   499  	return list
   500  }
   501  
   502  // ----------------------------------------------------------------------------
   503  // Types
   504  
   505  func (p *parser) parseType() ast.Expr {
   506  	if p.trace {
   507  		defer un(trace(p, "Type"))
   508  	}
   509  
   510  	typ := p.tryIdentOrType()
   511  
   512  	if typ == nil {
   513  		pos := p.pos
   514  		p.errorExpected(pos, "type")
   515  		p.advance(exprEnd)
   516  		return &ast.BadExpr{From: pos, To: p.pos}
   517  	}
   518  
   519  	return typ
   520  }
   521  
   522  func (p *parser) parseQualifiedIdent(ident *ast.Ident) ast.Expr {
   523  	if p.trace {
   524  		defer un(trace(p, "QualifiedIdent"))
   525  	}
   526  
   527  	typ := p.parseTypeName(ident)
   528  	if p.tok == token.LBRACK {
   529  		typ = p.parseTypeInstance(typ)
   530  	}
   531  
   532  	return typ
   533  }
   534  
   535  // If the result is an identifier, it is not resolved.
   536  func (p *parser) parseTypeName(ident *ast.Ident) ast.Expr {
   537  	if p.trace {
   538  		defer un(trace(p, "TypeName"))
   539  	}
   540  
   541  	if ident == nil {
   542  		ident = p.parseIdent()
   543  	}
   544  
   545  	if p.tok == token.PERIOD {
   546  		// ident is a package name
   547  		p.next()
   548  		sel := p.parseIdent()
   549  		return &ast.SelectorExpr{X: ident, Sel: sel}
   550  	}
   551  
   552  	return ident
   553  }
   554  
   555  // "[" has already been consumed, and lbrack is its position.
   556  // If len != nil it is the already consumed array length.
   557  func (p *parser) parseArrayType(lbrack token.Pos, len ast.Expr) *ast.ArrayType {
   558  	if p.trace {
   559  		defer un(trace(p, "ArrayType"))
   560  	}
   561  
   562  	if len == nil {
   563  		p.exprLev++
   564  		// always permit ellipsis for more fault-tolerant parsing
   565  		if p.tok == token.ELLIPSIS {
   566  			len = &ast.Ellipsis{Ellipsis: p.pos}
   567  			p.next()
   568  		} else if p.tok != token.RBRACK {
   569  			len = p.parseRhs()
   570  		}
   571  		p.exprLev--
   572  	}
   573  	if p.tok == token.COMMA {
   574  		// Trailing commas are accepted in type parameter
   575  		// lists but not in array type declarations.
   576  		// Accept for better error handling but complain.
   577  		p.error(p.pos, "unexpected comma; expecting ]")
   578  		p.next()
   579  	}
   580  	p.expect(token.RBRACK)
   581  	elt := p.parseType()
   582  	return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
   583  }
   584  
   585  func (p *parser) parseArrayFieldOrTypeInstance(x *ast.Ident) (*ast.Ident, ast.Expr) {
   586  	if p.trace {
   587  		defer un(trace(p, "ArrayFieldOrTypeInstance"))
   588  	}
   589  
   590  	lbrack := p.expect(token.LBRACK)
   591  	trailingComma := token.NoPos // if valid, the position of a trailing comma preceding the ']'
   592  	var args []ast.Expr
   593  	if p.tok != token.RBRACK {
   594  		p.exprLev++
   595  		args = append(args, p.parseRhs())
   596  		for p.tok == token.COMMA {
   597  			comma := p.pos
   598  			p.next()
   599  			if p.tok == token.RBRACK {
   600  				trailingComma = comma
   601  				break
   602  			}
   603  			args = append(args, p.parseRhs())
   604  		}
   605  		p.exprLev--
   606  	}
   607  	rbrack := p.expect(token.RBRACK)
   608  
   609  	if len(args) == 0 {
   610  		// x []E
   611  		elt := p.parseType()
   612  		return x, &ast.ArrayType{Lbrack: lbrack, Elt: elt}
   613  	}
   614  
   615  	// x [P]E or x[P]
   616  	if len(args) == 1 {
   617  		elt := p.tryIdentOrType()
   618  		if elt != nil {
   619  			// x [P]E
   620  			if trailingComma.IsValid() {
   621  				// Trailing commas are invalid in array type fields.
   622  				p.error(trailingComma, "unexpected comma; expecting ]")
   623  			}
   624  			return x, &ast.ArrayType{Lbrack: lbrack, Len: args[0], Elt: elt}
   625  		}
   626  	}
   627  
   628  	// x[P], x[P1, P2], ...
   629  	return nil, typeparams.PackIndexExpr(x, lbrack, args, rbrack)
   630  }
   631  
   632  func (p *parser) parseFieldDecl() *ast.Field {
   633  	if p.trace {
   634  		defer un(trace(p, "FieldDecl"))
   635  	}
   636  
   637  	doc := p.leadComment
   638  
   639  	var names []*ast.Ident
   640  	var typ ast.Expr
   641  	switch p.tok {
   642  	case token.IDENT:
   643  		name := p.parseIdent()
   644  		if p.tok == token.PERIOD || p.tok == token.STRING || p.tok == token.SEMICOLON || p.tok == token.RBRACE {
   645  			// embedded type
   646  			typ = name
   647  			if p.tok == token.PERIOD {
   648  				typ = p.parseQualifiedIdent(name)
   649  			}
   650  		} else {
   651  			// name1, name2, ... T
   652  			names = []*ast.Ident{name}
   653  			for p.tok == token.COMMA {
   654  				p.next()
   655  				names = append(names, p.parseIdent())
   656  			}
   657  			// Careful dance: We don't know if we have an embedded instantiated
   658  			// type T[P1, P2, ...] or a field T of array type []E or [P]E.
   659  			if len(names) == 1 && p.tok == token.LBRACK {
   660  				name, typ = p.parseArrayFieldOrTypeInstance(name)
   661  				if name == nil {
   662  					names = nil
   663  				}
   664  			} else {
   665  				// T P
   666  				typ = p.parseType()
   667  			}
   668  		}
   669  	case token.MUL:
   670  		star := p.pos
   671  		p.next()
   672  		if p.tok == token.LPAREN {
   673  			// *(T)
   674  			p.error(p.pos, "cannot parenthesize embedded type")
   675  			p.next()
   676  			typ = p.parseQualifiedIdent(nil)
   677  			// expect closing ')' but no need to complain if missing
   678  			if p.tok == token.RPAREN {
   679  				p.next()
   680  			}
   681  		} else {
   682  			// *T
   683  			typ = p.parseQualifiedIdent(nil)
   684  		}
   685  		typ = &ast.StarExpr{Star: star, X: typ}
   686  
   687  	case token.LPAREN:
   688  		p.error(p.pos, "cannot parenthesize embedded type")
   689  		p.next()
   690  		if p.tok == token.MUL {
   691  			// (*T)
   692  			star := p.pos
   693  			p.next()
   694  			typ = &ast.StarExpr{Star: star, X: p.parseQualifiedIdent(nil)}
   695  		} else {
   696  			// (T)
   697  			typ = p.parseQualifiedIdent(nil)
   698  		}
   699  		// expect closing ')' but no need to complain if missing
   700  		if p.tok == token.RPAREN {
   701  			p.next()
   702  		}
   703  
   704  	default:
   705  		pos := p.pos
   706  		p.errorExpected(pos, "field name or embedded type")
   707  		p.advance(exprEnd)
   708  		typ = &ast.BadExpr{From: pos, To: p.pos}
   709  	}
   710  
   711  	var tag *ast.BasicLit
   712  	if p.tok == token.STRING {
   713  		tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
   714  		p.next()
   715  	}
   716  
   717  	comment := p.expectSemi()
   718  
   719  	field := &ast.Field{Doc: doc, Names: names, Type: typ, Tag: tag, Comment: comment}
   720  	return field
   721  }
   722  
   723  func (p *parser) parseStructType() *ast.StructType {
   724  	if p.trace {
   725  		defer un(trace(p, "StructType"))
   726  	}
   727  
   728  	pos := p.expect(token.STRUCT)
   729  	lbrace := p.expect(token.LBRACE)
   730  	var list []*ast.Field
   731  	for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
   732  		// a field declaration cannot start with a '(' but we accept
   733  		// it here for more robust parsing and better error messages
   734  		// (parseFieldDecl will check and complain if necessary)
   735  		list = append(list, p.parseFieldDecl())
   736  	}
   737  	rbrace := p.expect(token.RBRACE)
   738  
   739  	return &ast.StructType{
   740  		Struct: pos,
   741  		Fields: &ast.FieldList{
   742  			Opening: lbrace,
   743  			List:    list,
   744  			Closing: rbrace,
   745  		},
   746  	}
   747  }
   748  
   749  func (p *parser) parsePointerType() *ast.StarExpr {
   750  	if p.trace {
   751  		defer un(trace(p, "PointerType"))
   752  	}
   753  
   754  	star := p.expect(token.MUL)
   755  	base := p.parseType()
   756  
   757  	return &ast.StarExpr{Star: star, X: base}
   758  }
   759  
   760  func (p *parser) parseDotsType() *ast.Ellipsis {
   761  	if p.trace {
   762  		defer un(trace(p, "DotsType"))
   763  	}
   764  
   765  	pos := p.expect(token.ELLIPSIS)
   766  	elt := p.parseType()
   767  
   768  	return &ast.Ellipsis{Ellipsis: pos, Elt: elt}
   769  }
   770  
   771  type field struct {
   772  	name *ast.Ident
   773  	typ  ast.Expr
   774  }
   775  
   776  func (p *parser) parseParamDecl(name *ast.Ident, typeSetsOK bool) (f field) {
   777  	// TODO(rFindley) refactor to be more similar to paramDeclOrNil in the syntax
   778  	// package
   779  	if p.trace {
   780  		defer un(trace(p, "ParamDeclOrNil"))
   781  	}
   782  
   783  	ptok := p.tok
   784  	if name != nil {
   785  		p.tok = token.IDENT // force token.IDENT case in switch below
   786  	} else if typeSetsOK && p.tok == token.TILDE {
   787  		// "~" ...
   788  		return field{nil, p.embeddedElem(nil)}
   789  	}
   790  
   791  	switch p.tok {
   792  	case token.IDENT:
   793  		// name
   794  		if name != nil {
   795  			f.name = name
   796  			p.tok = ptok
   797  		} else {
   798  			f.name = p.parseIdent()
   799  		}
   800  		switch p.tok {
   801  		case token.IDENT, token.MUL, token.ARROW, token.FUNC, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   802  			// name type
   803  			f.typ = p.parseType()
   804  
   805  		case token.LBRACK:
   806  			// name "[" type1, ..., typeN "]" or name "[" n "]" type
   807  			f.name, f.typ = p.parseArrayFieldOrTypeInstance(f.name)
   808  
   809  		case token.ELLIPSIS:
   810  			// name "..." type
   811  			f.typ = p.parseDotsType()
   812  			return // don't allow ...type "|" ...
   813  
   814  		case token.PERIOD:
   815  			// name "." ...
   816  			f.typ = p.parseQualifiedIdent(f.name)
   817  			f.name = nil
   818  
   819  		case token.TILDE:
   820  			if typeSetsOK {
   821  				f.typ = p.embeddedElem(nil)
   822  				return
   823  			}
   824  
   825  		case token.OR:
   826  			if typeSetsOK {
   827  				// name "|" typeset
   828  				f.typ = p.embeddedElem(f.name)
   829  				f.name = nil
   830  				return
   831  			}
   832  		}
   833  
   834  	case token.MUL, token.ARROW, token.FUNC, token.LBRACK, token.CHAN, token.MAP, token.STRUCT, token.INTERFACE, token.LPAREN:
   835  		// type
   836  		f.typ = p.parseType()
   837  
   838  	case token.ELLIPSIS:
   839  		// "..." type
   840  		// (always accepted)
   841  		f.typ = p.parseDotsType()
   842  		return // don't allow ...type "|" ...
   843  
   844  	default:
   845  		// TODO(rfindley): this is incorrect in the case of type parameter lists
   846  		//                 (should be "']'" in that case)
   847  		p.errorExpected(p.pos, "')'")
   848  		p.advance(exprEnd)
   849  	}
   850  
   851  	// [name] type "|"
   852  	if typeSetsOK && p.tok == token.OR && f.typ != nil {
   853  		f.typ = p.embeddedElem(f.typ)
   854  	}
   855  
   856  	return
   857  }
   858  
   859  func (p *parser) parseParameterList(name0 *ast.Ident, typ0 ast.Expr, closing token.Token) (params []*ast.Field) {
   860  	if p.trace {
   861  		defer un(trace(p, "ParameterList"))
   862  	}
   863  
   864  	// Type parameters are the only parameter list closed by ']'.
   865  	tparams := closing == token.RBRACK
   866  	// Type set notation is ok in type parameter lists.
   867  	typeSetsOK := tparams
   868  
   869  	pos := p.pos
   870  	if name0 != nil {
   871  		pos = name0.Pos()
   872  	}
   873  
   874  	var list []field
   875  	var named int // number of parameters that have an explicit name and type
   876  
   877  	for name0 != nil || p.tok != closing && p.tok != token.EOF {
   878  		var par field
   879  		if typ0 != nil {
   880  			if typeSetsOK {
   881  				typ0 = p.embeddedElem(typ0)
   882  			}
   883  			par = field{name0, typ0}
   884  		} else {
   885  			par = p.parseParamDecl(name0, typeSetsOK)
   886  		}
   887  		name0 = nil // 1st name was consumed if present
   888  		typ0 = nil  // 1st typ was consumed if present
   889  		if par.name != nil || par.typ != nil {
   890  			list = append(list, par)
   891  			if par.name != nil && par.typ != nil {
   892  				named++
   893  			}
   894  		}
   895  		if !p.atComma("parameter list", closing) {
   896  			break
   897  		}
   898  		p.next()
   899  	}
   900  
   901  	if len(list) == 0 {
   902  		return // not uncommon
   903  	}
   904  
   905  	// TODO(gri) parameter distribution and conversion to []*ast.Field
   906  	//           can be combined and made more efficient
   907  
   908  	// distribute parameter types
   909  	if named == 0 {
   910  		// all unnamed => found names are type names
   911  		for i := 0; i < len(list); i++ {
   912  			par := &list[i]
   913  			if typ := par.name; typ != nil {
   914  				par.typ = typ
   915  				par.name = nil
   916  			}
   917  		}
   918  		if tparams {
   919  			p.error(pos, "type parameters must be named")
   920  		}
   921  	} else if named != len(list) {
   922  		// some named => all must be named
   923  		ok := true
   924  		var typ ast.Expr
   925  		missingName := pos
   926  		for i := len(list) - 1; i >= 0; i-- {
   927  			if par := &list[i]; par.typ != nil {
   928  				typ = par.typ
   929  				if par.name == nil {
   930  					ok = false
   931  					missingName = par.typ.Pos()
   932  					n := ast.NewIdent("_")
   933  					n.NamePos = typ.Pos() // correct position
   934  					par.name = n
   935  				}
   936  			} else if typ != nil {
   937  				par.typ = typ
   938  			} else {
   939  				// par.typ == nil && typ == nil => we only have a par.name
   940  				ok = false
   941  				missingName = par.name.Pos()
   942  				par.typ = &ast.BadExpr{From: par.name.Pos(), To: p.pos}
   943  			}
   944  		}
   945  		if !ok {
   946  			if tparams {
   947  				p.error(missingName, "type parameters must be named")
   948  			} else {
   949  				p.error(pos, "mixed named and unnamed parameters")
   950  			}
   951  		}
   952  	}
   953  
   954  	// convert list []*ast.Field
   955  	if named == 0 {
   956  		// parameter list consists of types only
   957  		for _, par := range list {
   958  			assert(par.typ != nil, "nil type in unnamed parameter list")
   959  			params = append(params, &ast.Field{Type: par.typ})
   960  		}
   961  		return
   962  	}
   963  
   964  	// parameter list consists of named parameters with types
   965  	var names []*ast.Ident
   966  	var typ ast.Expr
   967  	addParams := func() {
   968  		assert(typ != nil, "nil type in named parameter list")
   969  		field := &ast.Field{Names: names, Type: typ}
   970  		params = append(params, field)
   971  		names = nil
   972  	}
   973  	for _, par := range list {
   974  		if par.typ != typ {
   975  			if len(names) > 0 {
   976  				addParams()
   977  			}
   978  			typ = par.typ
   979  		}
   980  		names = append(names, par.name)
   981  	}
   982  	if len(names) > 0 {
   983  		addParams()
   984  	}
   985  	return
   986  }
   987  
   988  func (p *parser) parseParameters(acceptTParams bool) (tparams, params *ast.FieldList) {
   989  	if p.trace {
   990  		defer un(trace(p, "Parameters"))
   991  	}
   992  
   993  	if acceptTParams && p.tok == token.LBRACK {
   994  		opening := p.pos
   995  		p.next()
   996  		// [T any](params) syntax
   997  		list := p.parseParameterList(nil, nil, token.RBRACK)
   998  		rbrack := p.expect(token.RBRACK)
   999  		tparams = &ast.FieldList{Opening: opening, List: list, Closing: rbrack}
  1000  		// Type parameter lists must not be empty.
  1001  		if tparams.NumFields() == 0 {
  1002  			p.error(tparams.Closing, "empty type parameter list")
  1003  			tparams = nil // avoid follow-on errors
  1004  		}
  1005  	}
  1006  
  1007  	opening := p.expect(token.LPAREN)
  1008  
  1009  	var fields []*ast.Field
  1010  	if p.tok != token.RPAREN {
  1011  		fields = p.parseParameterList(nil, nil, token.RPAREN)
  1012  	}
  1013  
  1014  	rparen := p.expect(token.RPAREN)
  1015  	params = &ast.FieldList{Opening: opening, List: fields, Closing: rparen}
  1016  
  1017  	return
  1018  }
  1019  
  1020  func (p *parser) parseResult() *ast.FieldList {
  1021  	if p.trace {
  1022  		defer un(trace(p, "Result"))
  1023  	}
  1024  
  1025  	if p.tok == token.LPAREN {
  1026  		_, results := p.parseParameters(false)
  1027  		return results
  1028  	}
  1029  
  1030  	typ := p.tryIdentOrType()
  1031  	if typ != nil {
  1032  		list := make([]*ast.Field, 1)
  1033  		list[0] = &ast.Field{Type: typ}
  1034  		return &ast.FieldList{List: list}
  1035  	}
  1036  
  1037  	return nil
  1038  }
  1039  
  1040  func (p *parser) parseFuncType() *ast.FuncType {
  1041  	if p.trace {
  1042  		defer un(trace(p, "FuncType"))
  1043  	}
  1044  
  1045  	pos := p.expect(token.FUNC)
  1046  	tparams, params := p.parseParameters(true)
  1047  	if tparams != nil {
  1048  		p.error(tparams.Pos(), "function type must have no type parameters")
  1049  	}
  1050  	results := p.parseResult()
  1051  
  1052  	return &ast.FuncType{Func: pos, Params: params, Results: results}
  1053  }
  1054  
  1055  func (p *parser) parseMethodSpec() *ast.Field {
  1056  	if p.trace {
  1057  		defer un(trace(p, "MethodSpec"))
  1058  	}
  1059  
  1060  	doc := p.leadComment
  1061  	var idents []*ast.Ident
  1062  	var typ ast.Expr
  1063  	x := p.parseTypeName(nil)
  1064  	if ident, _ := x.(*ast.Ident); ident != nil {
  1065  		switch {
  1066  		case p.tok == token.LBRACK:
  1067  			// generic method or embedded instantiated type
  1068  			lbrack := p.pos
  1069  			p.next()
  1070  			p.exprLev++
  1071  			x := p.parseExpr()
  1072  			p.exprLev--
  1073  			if name0, _ := x.(*ast.Ident); name0 != nil && p.tok != token.COMMA && p.tok != token.RBRACK {
  1074  				// generic method m[T any]
  1075  				//
  1076  				// Interface methods do not have type parameters. We parse them for a
  1077  				// better error message and improved error recovery.
  1078  				_ = p.parseParameterList(name0, nil, token.RBRACK)
  1079  				_ = p.expect(token.RBRACK)
  1080  				p.error(lbrack, "interface method must have no type parameters")
  1081  
  1082  				// TODO(rfindley) refactor to share code with parseFuncType.
  1083  				_, params := p.parseParameters(false)
  1084  				results := p.parseResult()
  1085  				idents = []*ast.Ident{ident}
  1086  				typ = &ast.FuncType{
  1087  					Func:    token.NoPos,
  1088  					Params:  params,
  1089  					Results: results,
  1090  				}
  1091  			} else {
  1092  				// embedded instantiated type
  1093  				// TODO(rfindley) should resolve all identifiers in x.
  1094  				list := []ast.Expr{x}
  1095  				if p.atComma("type argument list", token.RBRACK) {
  1096  					p.exprLev++
  1097  					p.next()
  1098  					for p.tok != token.RBRACK && p.tok != token.EOF {
  1099  						list = append(list, p.parseType())
  1100  						if !p.atComma("type argument list", token.RBRACK) {
  1101  							break
  1102  						}
  1103  						p.next()
  1104  					}
  1105  					p.exprLev--
  1106  				}
  1107  				rbrack := p.expectClosing(token.RBRACK, "type argument list")
  1108  				typ = typeparams.PackIndexExpr(ident, lbrack, list, rbrack)
  1109  			}
  1110  		case p.tok == token.LPAREN:
  1111  			// ordinary method
  1112  			// TODO(rfindley) refactor to share code with parseFuncType.
  1113  			_, params := p.parseParameters(false)
  1114  			results := p.parseResult()
  1115  			idents = []*ast.Ident{ident}
  1116  			typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
  1117  		default:
  1118  			// embedded type
  1119  			typ = x
  1120  		}
  1121  	} else {
  1122  		// embedded, possibly instantiated type
  1123  		typ = x
  1124  		if p.tok == token.LBRACK {
  1125  			// embedded instantiated interface
  1126  			typ = p.parseTypeInstance(typ)
  1127  		}
  1128  	}
  1129  
  1130  	// Comment is added at the callsite: the field below may joined with
  1131  	// additional type specs using '|'.
  1132  	// TODO(rfindley) this should be refactored.
  1133  	// TODO(rfindley) add more tests for comment handling.
  1134  	return &ast.Field{Doc: doc, Names: idents, Type: typ}
  1135  }
  1136  
  1137  func (p *parser) embeddedElem(x ast.Expr) ast.Expr {
  1138  	if p.trace {
  1139  		defer un(trace(p, "EmbeddedElem"))
  1140  	}
  1141  	if x == nil {
  1142  		x = p.embeddedTerm()
  1143  	}
  1144  	for p.tok == token.OR {
  1145  		t := new(ast.BinaryExpr)
  1146  		t.OpPos = p.pos
  1147  		t.Op = token.OR
  1148  		p.next()
  1149  		t.X = x
  1150  		t.Y = p.embeddedTerm()
  1151  		x = t
  1152  	}
  1153  	return x
  1154  }
  1155  
  1156  func (p *parser) embeddedTerm() ast.Expr {
  1157  	if p.trace {
  1158  		defer un(trace(p, "EmbeddedTerm"))
  1159  	}
  1160  	if p.tok == token.TILDE {
  1161  		t := new(ast.UnaryExpr)
  1162  		t.OpPos = p.pos
  1163  		t.Op = token.TILDE
  1164  		p.next()
  1165  		t.X = p.parseType()
  1166  		return t
  1167  	}
  1168  
  1169  	t := p.tryIdentOrType()
  1170  	if t == nil {
  1171  		pos := p.pos
  1172  		p.errorExpected(pos, "~ term or type")
  1173  		p.advance(exprEnd)
  1174  		return &ast.BadExpr{From: pos, To: p.pos}
  1175  	}
  1176  
  1177  	return t
  1178  }
  1179  
  1180  func (p *parser) parseInterfaceType() *ast.InterfaceType {
  1181  	if p.trace {
  1182  		defer un(trace(p, "InterfaceType"))
  1183  	}
  1184  
  1185  	pos := p.expect(token.INTERFACE)
  1186  	lbrace := p.expect(token.LBRACE)
  1187  
  1188  	var list []*ast.Field
  1189  
  1190  parseElements:
  1191  	for {
  1192  		switch {
  1193  		case p.tok == token.IDENT:
  1194  			f := p.parseMethodSpec()
  1195  			if f.Names == nil {
  1196  				f.Type = p.embeddedElem(f.Type)
  1197  			}
  1198  			f.Comment = p.expectSemi()
  1199  			list = append(list, f)
  1200  		case p.tok == token.TILDE:
  1201  			typ := p.embeddedElem(nil)
  1202  			comment := p.expectSemi()
  1203  			list = append(list, &ast.Field{Type: typ, Comment: comment})
  1204  		default:
  1205  			if t := p.tryIdentOrType(); t != nil {
  1206  				typ := p.embeddedElem(t)
  1207  				comment := p.expectSemi()
  1208  				list = append(list, &ast.Field{Type: typ, Comment: comment})
  1209  			} else {
  1210  				break parseElements
  1211  			}
  1212  		}
  1213  	}
  1214  
  1215  	// TODO(rfindley): the error produced here could be improved, since we could
  1216  	// accept a identifier, 'type', or a '}' at this point.
  1217  	rbrace := p.expect(token.RBRACE)
  1218  
  1219  	return &ast.InterfaceType{
  1220  		Interface: pos,
  1221  		Methods: &ast.FieldList{
  1222  			Opening: lbrace,
  1223  			List:    list,
  1224  			Closing: rbrace,
  1225  		},
  1226  	}
  1227  }
  1228  
  1229  func (p *parser) parseMapType() *ast.MapType {
  1230  	if p.trace {
  1231  		defer un(trace(p, "MapType"))
  1232  	}
  1233  
  1234  	pos := p.expect(token.MAP)
  1235  	p.expect(token.LBRACK)
  1236  	key := p.parseType()
  1237  	p.expect(token.RBRACK)
  1238  	value := p.parseType()
  1239  
  1240  	return &ast.MapType{Map: pos, Key: key, Value: value}
  1241  }
  1242  
  1243  func (p *parser) parseChanType() *ast.ChanType {
  1244  	if p.trace {
  1245  		defer un(trace(p, "ChanType"))
  1246  	}
  1247  
  1248  	pos := p.pos
  1249  	dir := ast.SEND | ast.RECV
  1250  	var arrow token.Pos
  1251  	if p.tok == token.CHAN {
  1252  		p.next()
  1253  		if p.tok == token.ARROW {
  1254  			arrow = p.pos
  1255  			p.next()
  1256  			dir = ast.SEND
  1257  		}
  1258  	} else {
  1259  		arrow = p.expect(token.ARROW)
  1260  		p.expect(token.CHAN)
  1261  		dir = ast.RECV
  1262  	}
  1263  	value := p.parseType()
  1264  
  1265  	return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
  1266  }
  1267  
  1268  func (p *parser) parseTypeInstance(typ ast.Expr) ast.Expr {
  1269  	if p.trace {
  1270  		defer un(trace(p, "TypeInstance"))
  1271  	}
  1272  
  1273  	opening := p.expect(token.LBRACK)
  1274  	p.exprLev++
  1275  	var list []ast.Expr
  1276  	for p.tok != token.RBRACK && p.tok != token.EOF {
  1277  		list = append(list, p.parseType())
  1278  		if !p.atComma("type argument list", token.RBRACK) {
  1279  			break
  1280  		}
  1281  		p.next()
  1282  	}
  1283  	p.exprLev--
  1284  
  1285  	closing := p.expectClosing(token.RBRACK, "type argument list")
  1286  
  1287  	if len(list) == 0 {
  1288  		p.errorExpected(closing, "type argument list")
  1289  		return &ast.IndexExpr{
  1290  			X:      typ,
  1291  			Lbrack: opening,
  1292  			Index:  &ast.BadExpr{From: opening + 1, To: closing},
  1293  			Rbrack: closing,
  1294  		}
  1295  	}
  1296  
  1297  	return typeparams.PackIndexExpr(typ, opening, list, closing)
  1298  }
  1299  
  1300  func (p *parser) tryIdentOrType() ast.Expr {
  1301  	defer decNestLev(incNestLev(p))
  1302  
  1303  	switch p.tok {
  1304  	case token.IDENT:
  1305  		typ := p.parseTypeName(nil)
  1306  		if p.tok == token.LBRACK {
  1307  			typ = p.parseTypeInstance(typ)
  1308  		}
  1309  		return typ
  1310  	case token.LBRACK:
  1311  		lbrack := p.expect(token.LBRACK)
  1312  		return p.parseArrayType(lbrack, nil)
  1313  	case token.STRUCT:
  1314  		return p.parseStructType()
  1315  	case token.MUL:
  1316  		return p.parsePointerType()
  1317  	case token.FUNC:
  1318  		return p.parseFuncType()
  1319  	case token.INTERFACE:
  1320  		return p.parseInterfaceType()
  1321  	case token.MAP:
  1322  		return p.parseMapType()
  1323  	case token.CHAN, token.ARROW:
  1324  		return p.parseChanType()
  1325  	case token.LPAREN:
  1326  		lparen := p.pos
  1327  		p.next()
  1328  		typ := p.parseType()
  1329  		rparen := p.expect(token.RPAREN)
  1330  		return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
  1331  	}
  1332  
  1333  	// no type found
  1334  	return nil
  1335  }
  1336  
  1337  // ----------------------------------------------------------------------------
  1338  // Blocks
  1339  
  1340  func (p *parser) parseStmtList() (list []ast.Stmt) {
  1341  	if p.trace {
  1342  		defer un(trace(p, "StatementList"))
  1343  	}
  1344  
  1345  	for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
  1346  		list = append(list, p.parseStmt())
  1347  	}
  1348  
  1349  	return
  1350  }
  1351  
  1352  func (p *parser) parseBody() *ast.BlockStmt {
  1353  	if p.trace {
  1354  		defer un(trace(p, "Body"))
  1355  	}
  1356  
  1357  	lbrace := p.expect(token.LBRACE)
  1358  	list := p.parseStmtList()
  1359  	rbrace := p.expect2(token.RBRACE)
  1360  
  1361  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1362  }
  1363  
  1364  func (p *parser) parseBlockStmt() *ast.BlockStmt {
  1365  	if p.trace {
  1366  		defer un(trace(p, "BlockStmt"))
  1367  	}
  1368  
  1369  	lbrace := p.expect(token.LBRACE)
  1370  	list := p.parseStmtList()
  1371  	rbrace := p.expect2(token.RBRACE)
  1372  
  1373  	return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  1374  }
  1375  
  1376  // ----------------------------------------------------------------------------
  1377  // Expressions
  1378  
  1379  func (p *parser) parseFuncTypeOrLit() ast.Expr {
  1380  	if p.trace {
  1381  		defer un(trace(p, "FuncTypeOrLit"))
  1382  	}
  1383  
  1384  	typ := p.parseFuncType()
  1385  	if p.tok != token.LBRACE {
  1386  		// function type only
  1387  		return typ
  1388  	}
  1389  
  1390  	p.exprLev++
  1391  	body := p.parseBody()
  1392  	p.exprLev--
  1393  
  1394  	return &ast.FuncLit{Type: typ, Body: body}
  1395  }
  1396  
  1397  // parseOperand may return an expression or a raw type (incl. array
  1398  // types of the form [...]T). Callers must verify the result.
  1399  func (p *parser) parseOperand() ast.Expr {
  1400  	if p.trace {
  1401  		defer un(trace(p, "Operand"))
  1402  	}
  1403  
  1404  	switch p.tok {
  1405  	case token.IDENT:
  1406  		x := p.parseIdent()
  1407  		return x
  1408  
  1409  	case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
  1410  		x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
  1411  		p.next()
  1412  		return x
  1413  
  1414  	case token.LPAREN:
  1415  		lparen := p.pos
  1416  		p.next()
  1417  		p.exprLev++
  1418  		x := p.parseRhs() // types may be parenthesized: (some type)
  1419  		p.exprLev--
  1420  		rparen := p.expect(token.RPAREN)
  1421  		return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}
  1422  
  1423  	case token.FUNC:
  1424  		return p.parseFuncTypeOrLit()
  1425  	}
  1426  
  1427  	if typ := p.tryIdentOrType(); typ != nil { // do not consume trailing type parameters
  1428  		// could be type for composite literal or conversion
  1429  		_, isIdent := typ.(*ast.Ident)
  1430  		assert(!isIdent, "type cannot be identifier")
  1431  		return typ
  1432  	}
  1433  
  1434  	// we have an error
  1435  	pos := p.pos
  1436  	p.errorExpected(pos, "operand")
  1437  	p.advance(stmtStart)
  1438  	return &ast.BadExpr{From: pos, To: p.pos}
  1439  }
  1440  
  1441  func (p *parser) parseSelector(x ast.Expr) ast.Expr {
  1442  	if p.trace {
  1443  		defer un(trace(p, "Selector"))
  1444  	}
  1445  
  1446  	sel := p.parseIdent()
  1447  
  1448  	return &ast.SelectorExpr{X: x, Sel: sel}
  1449  }
  1450  
  1451  func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
  1452  	if p.trace {
  1453  		defer un(trace(p, "TypeAssertion"))
  1454  	}
  1455  
  1456  	lparen := p.expect(token.LPAREN)
  1457  	var typ ast.Expr
  1458  	if p.tok == token.TYPE {
  1459  		// type switch: typ == nil
  1460  		p.next()
  1461  	} else {
  1462  		typ = p.parseType()
  1463  	}
  1464  	rparen := p.expect(token.RPAREN)
  1465  
  1466  	return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
  1467  }
  1468  
  1469  func (p *parser) parseIndexOrSliceOrInstance(x ast.Expr) ast.Expr {
  1470  	if p.trace {
  1471  		defer un(trace(p, "parseIndexOrSliceOrInstance"))
  1472  	}
  1473  
  1474  	lbrack := p.expect(token.LBRACK)
  1475  	if p.tok == token.RBRACK {
  1476  		// empty index, slice or index expressions are not permitted;
  1477  		// accept them for parsing tolerance, but complain
  1478  		p.errorExpected(p.pos, "operand")
  1479  		rbrack := p.pos
  1480  		p.next()
  1481  		return &ast.IndexExpr{
  1482  			X:      x,
  1483  			Lbrack: lbrack,
  1484  			Index:  &ast.BadExpr{From: rbrack, To: rbrack},
  1485  			Rbrack: rbrack,
  1486  		}
  1487  	}
  1488  	p.exprLev++
  1489  
  1490  	const N = 3 // change the 3 to 2 to disable 3-index slices
  1491  	var args []ast.Expr
  1492  	var index [N]ast.Expr
  1493  	var colons [N - 1]token.Pos
  1494  	if p.tok != token.COLON {
  1495  		// We can't know if we have an index expression or a type instantiation;
  1496  		// so even if we see a (named) type we are not going to be in type context.
  1497  		index[0] = p.parseRhs()
  1498  	}
  1499  	ncolons := 0
  1500  	switch p.tok {
  1501  	case token.COLON:
  1502  		// slice expression
  1503  		for p.tok == token.COLON && ncolons < len(colons) {
  1504  			colons[ncolons] = p.pos
  1505  			ncolons++
  1506  			p.next()
  1507  			if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
  1508  				index[ncolons] = p.parseRhs()
  1509  			}
  1510  		}
  1511  	case token.COMMA:
  1512  		// instance expression
  1513  		args = append(args, index[0])
  1514  		for p.tok == token.COMMA {
  1515  			p.next()
  1516  			if p.tok != token.RBRACK && p.tok != token.EOF {
  1517  				args = append(args, p.parseType())
  1518  			}
  1519  		}
  1520  	}
  1521  
  1522  	p.exprLev--
  1523  	rbrack := p.expect(token.RBRACK)
  1524  
  1525  	if ncolons > 0 {
  1526  		// slice expression
  1527  		slice3 := false
  1528  		if ncolons == 2 {
  1529  			slice3 = true
  1530  			// Check presence of middle and final index here rather than during type-checking
  1531  			// to prevent erroneous programs from passing through gofmt (was issue 7305).
  1532  			if index[1] == nil {
  1533  				p.error(colons[0], "middle index required in 3-index slice")
  1534  				index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
  1535  			}
  1536  			if index[2] == nil {
  1537  				p.error(colons[1], "final index required in 3-index slice")
  1538  				index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
  1539  			}
  1540  		}
  1541  		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
  1542  	}
  1543  
  1544  	if len(args) == 0 {
  1545  		// index expression
  1546  		return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
  1547  	}
  1548  
  1549  	// instance expression
  1550  	return typeparams.PackIndexExpr(x, lbrack, args, rbrack)
  1551  }
  1552  
  1553  func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
  1554  	if p.trace {
  1555  		defer un(trace(p, "CallOrConversion"))
  1556  	}
  1557  
  1558  	lparen := p.expect(token.LPAREN)
  1559  	p.exprLev++
  1560  	var list []ast.Expr
  1561  	var ellipsis token.Pos
  1562  	for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
  1563  		list = append(list, p.parseRhs()) // builtins may expect a type: make(some type, ...)
  1564  		if p.tok == token.ELLIPSIS {
  1565  			ellipsis = p.pos
  1566  			p.next()
  1567  		}
  1568  		if !p.atComma("argument list", token.RPAREN) {
  1569  			break
  1570  		}
  1571  		p.next()
  1572  	}
  1573  	p.exprLev--
  1574  	rparen := p.expectClosing(token.RPAREN, "argument list")
  1575  
  1576  	return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
  1577  }
  1578  
  1579  func (p *parser) parseValue() ast.Expr {
  1580  	if p.trace {
  1581  		defer un(trace(p, "Element"))
  1582  	}
  1583  
  1584  	if p.tok == token.LBRACE {
  1585  		return p.parseLiteralValue(nil)
  1586  	}
  1587  
  1588  	x := p.parseExpr()
  1589  
  1590  	return x
  1591  }
  1592  
  1593  func (p *parser) parseElement() ast.Expr {
  1594  	if p.trace {
  1595  		defer un(trace(p, "Element"))
  1596  	}
  1597  
  1598  	x := p.parseValue()
  1599  	if p.tok == token.COLON {
  1600  		colon := p.pos
  1601  		p.next()
  1602  		x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue()}
  1603  	}
  1604  
  1605  	return x
  1606  }
  1607  
  1608  func (p *parser) parseElementList() (list []ast.Expr) {
  1609  	if p.trace {
  1610  		defer un(trace(p, "ElementList"))
  1611  	}
  1612  
  1613  	for p.tok != token.RBRACE && p.tok != token.EOF {
  1614  		list = append(list, p.parseElement())
  1615  		if !p.atComma("composite literal", token.RBRACE) {
  1616  			break
  1617  		}
  1618  		p.next()
  1619  	}
  1620  
  1621  	return
  1622  }
  1623  
  1624  func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
  1625  	if p.trace {
  1626  		defer un(trace(p, "LiteralValue"))
  1627  	}
  1628  
  1629  	lbrace := p.expect(token.LBRACE)
  1630  	var elts []ast.Expr
  1631  	p.exprLev++
  1632  	if p.tok != token.RBRACE {
  1633  		elts = p.parseElementList()
  1634  	}
  1635  	p.exprLev--
  1636  	rbrace := p.expectClosing(token.RBRACE, "composite literal")
  1637  	return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
  1638  }
  1639  
  1640  // If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
  1641  func unparen(x ast.Expr) ast.Expr {
  1642  	if p, isParen := x.(*ast.ParenExpr); isParen {
  1643  		x = unparen(p.X)
  1644  	}
  1645  	return x
  1646  }
  1647  
  1648  func (p *parser) parsePrimaryExpr(x ast.Expr) ast.Expr {
  1649  	if p.trace {
  1650  		defer un(trace(p, "PrimaryExpr"))
  1651  	}
  1652  
  1653  	if x == nil {
  1654  		x = p.parseOperand()
  1655  	}
  1656  	// We track the nesting here rather than at the entry for the function,
  1657  	// since it can iteratively produce a nested output, and we want to
  1658  	// limit how deep a structure we generate.
  1659  	var n int
  1660  	defer func() { p.nestLev -= n }()
  1661  	for n = 1; ; n++ {
  1662  		incNestLev(p)
  1663  		switch p.tok {
  1664  		case token.PERIOD:
  1665  			p.next()
  1666  			switch p.tok {
  1667  			case token.IDENT:
  1668  				x = p.parseSelector(x)
  1669  			case token.LPAREN:
  1670  				x = p.parseTypeAssertion(x)
  1671  			default:
  1672  				pos := p.pos
  1673  				p.errorExpected(pos, "selector or type assertion")
  1674  				// TODO(rFindley) The check for token.RBRACE below is a targeted fix
  1675  				//                to error recovery sufficient to make the x/tools tests to
  1676  				//                pass with the new parsing logic introduced for type
  1677  				//                parameters. Remove this once error recovery has been
  1678  				//                more generally reconsidered.
  1679  				if p.tok != token.RBRACE {
  1680  					p.next() // make progress
  1681  				}
  1682  				sel := &ast.Ident{NamePos: pos, Name: "_"}
  1683  				x = &ast.SelectorExpr{X: x, Sel: sel}
  1684  			}
  1685  		case token.LBRACK:
  1686  			x = p.parseIndexOrSliceOrInstance(x)
  1687  		case token.LPAREN:
  1688  			x = p.parseCallOrConversion(x)
  1689  		case token.LBRACE:
  1690  			// operand may have returned a parenthesized complit
  1691  			// type; accept it but complain if we have a complit
  1692  			t := unparen(x)
  1693  			// determine if '{' belongs to a composite literal or a block statement
  1694  			switch t.(type) {
  1695  			case *ast.BadExpr, *ast.Ident, *ast.SelectorExpr:
  1696  				if p.exprLev < 0 {
  1697  					return x
  1698  				}
  1699  				// x is possibly a composite literal type
  1700  			case *ast.IndexExpr, *ast.IndexListExpr:
  1701  				if p.exprLev < 0 {
  1702  					return x
  1703  				}
  1704  				// x is possibly a composite literal type
  1705  			case *ast.ArrayType, *ast.StructType, *ast.MapType:
  1706  				// x is a composite literal type
  1707  			default:
  1708  				return x
  1709  			}
  1710  			if t != x {
  1711  				p.error(t.Pos(), "cannot parenthesize type in composite literal")
  1712  				// already progressed, no need to advance
  1713  			}
  1714  			x = p.parseLiteralValue(x)
  1715  		default:
  1716  			return x
  1717  		}
  1718  	}
  1719  }
  1720  
  1721  func (p *parser) parseUnaryExpr() ast.Expr {
  1722  	defer decNestLev(incNestLev(p))
  1723  
  1724  	if p.trace {
  1725  		defer un(trace(p, "UnaryExpr"))
  1726  	}
  1727  
  1728  	switch p.tok {
  1729  	case token.ADD, token.SUB, token.NOT, token.XOR, token.AND, token.TILDE:
  1730  		pos, op := p.pos, p.tok
  1731  		p.next()
  1732  		x := p.parseUnaryExpr()
  1733  		return &ast.UnaryExpr{OpPos: pos, Op: op, X: x}
  1734  
  1735  	case token.ARROW:
  1736  		// channel type or receive expression
  1737  		arrow := p.pos
  1738  		p.next()
  1739  
  1740  		// If the next token is token.CHAN we still don't know if it
  1741  		// is a channel type or a receive operation - we only know
  1742  		// once we have found the end of the unary expression. There
  1743  		// are two cases:
  1744  		//
  1745  		//   <- type  => (<-type) must be channel type
  1746  		//   <- expr  => <-(expr) is a receive from an expression
  1747  		//
  1748  		// In the first case, the arrow must be re-associated with
  1749  		// the channel type parsed already:
  1750  		//
  1751  		//   <- (chan type)    =>  (<-chan type)
  1752  		//   <- (chan<- type)  =>  (<-chan (<-type))
  1753  
  1754  		x := p.parseUnaryExpr()
  1755  
  1756  		// determine which case we have
  1757  		if typ, ok := x.(*ast.ChanType); ok {
  1758  			// (<-type)
  1759  
  1760  			// re-associate position info and <-
  1761  			dir := ast.SEND
  1762  			for ok && dir == ast.SEND {
  1763  				if typ.Dir == ast.RECV {
  1764  					// error: (<-type) is (<-(<-chan T))
  1765  					p.errorExpected(typ.Arrow, "'chan'")
  1766  				}
  1767  				arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
  1768  				dir, typ.Dir = typ.Dir, ast.RECV
  1769  				typ, ok = typ.Value.(*ast.ChanType)
  1770  			}
  1771  			if dir == ast.SEND {
  1772  				p.errorExpected(arrow, "channel type")
  1773  			}
  1774  
  1775  			return x
  1776  		}
  1777  
  1778  		// <-(expr)
  1779  		return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: x}
  1780  
  1781  	case token.MUL:
  1782  		// pointer type or unary "*" expression
  1783  		pos := p.pos
  1784  		p.next()
  1785  		x := p.parseUnaryExpr()
  1786  		return &ast.StarExpr{Star: pos, X: x}
  1787  	}
  1788  
  1789  	return p.parsePrimaryExpr(nil)
  1790  }
  1791  
  1792  func (p *parser) tokPrec() (token.Token, int) {
  1793  	tok := p.tok
  1794  	if p.inRhs && tok == token.ASSIGN {
  1795  		tok = token.EQL
  1796  	}
  1797  	return tok, tok.Precedence()
  1798  }
  1799  
  1800  // parseBinaryExpr parses a (possibly) binary expression.
  1801  // If x is non-nil, it is used as the left operand.
  1802  //
  1803  // TODO(rfindley): parseBinaryExpr has become overloaded. Consider refactoring.
  1804  func (p *parser) parseBinaryExpr(x ast.Expr, prec1 int) ast.Expr {
  1805  	if p.trace {
  1806  		defer un(trace(p, "BinaryExpr"))
  1807  	}
  1808  
  1809  	if x == nil {
  1810  		x = p.parseUnaryExpr()
  1811  	}
  1812  	// We track the nesting here rather than at the entry for the function,
  1813  	// since it can iteratively produce a nested output, and we want to
  1814  	// limit how deep a structure we generate.
  1815  	var n int
  1816  	defer func() { p.nestLev -= n }()
  1817  	for n = 1; ; n++ {
  1818  		incNestLev(p)
  1819  		op, oprec := p.tokPrec()
  1820  		if oprec < prec1 {
  1821  			return x
  1822  		}
  1823  		pos := p.expect(op)
  1824  		y := p.parseBinaryExpr(nil, oprec+1)
  1825  		x = &ast.BinaryExpr{X: x, OpPos: pos, Op: op, Y: y}
  1826  	}
  1827  }
  1828  
  1829  // The result may be a type or even a raw type ([...]int).
  1830  func (p *parser) parseExpr() ast.Expr {
  1831  	if p.trace {
  1832  		defer un(trace(p, "Expression"))
  1833  	}
  1834  
  1835  	return p.parseBinaryExpr(nil, token.LowestPrec+1)
  1836  }
  1837  
  1838  func (p *parser) parseRhs() ast.Expr {
  1839  	old := p.inRhs
  1840  	p.inRhs = true
  1841  	x := p.parseExpr()
  1842  	p.inRhs = old
  1843  	return x
  1844  }
  1845  
  1846  // ----------------------------------------------------------------------------
  1847  // Statements
  1848  
  1849  // Parsing modes for parseSimpleStmt.
  1850  const (
  1851  	basic = iota
  1852  	labelOk
  1853  	rangeOk
  1854  )
  1855  
  1856  // parseSimpleStmt returns true as 2nd result if it parsed the assignment
  1857  // of a range clause (with mode == rangeOk). The returned statement is an
  1858  // assignment with a right-hand side that is a single unary expression of
  1859  // the form "range x". No guarantees are given for the left-hand side.
  1860  func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
  1861  	if p.trace {
  1862  		defer un(trace(p, "SimpleStmt"))
  1863  	}
  1864  
  1865  	x := p.parseList(false)
  1866  
  1867  	switch p.tok {
  1868  	case
  1869  		token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
  1870  		token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
  1871  		token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
  1872  		token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
  1873  		// assignment statement, possibly part of a range clause
  1874  		pos, tok := p.pos, p.tok
  1875  		p.next()
  1876  		var y []ast.Expr
  1877  		isRange := false
  1878  		if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
  1879  			pos := p.pos
  1880  			p.next()
  1881  			y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  1882  			isRange = true
  1883  		} else {
  1884  			y = p.parseList(true)
  1885  		}
  1886  		return &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}, isRange
  1887  	}
  1888  
  1889  	if len(x) > 1 {
  1890  		p.errorExpected(x[0].Pos(), "1 expression")
  1891  		// continue with first expression
  1892  	}
  1893  
  1894  	switch p.tok {
  1895  	case token.COLON:
  1896  		// labeled statement
  1897  		colon := p.pos
  1898  		p.next()
  1899  		if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
  1900  			// Go spec: The scope of a label is the body of the function
  1901  			// in which it is declared and excludes the body of any nested
  1902  			// function.
  1903  			stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
  1904  			return stmt, false
  1905  		}
  1906  		// The label declaration typically starts at x[0].Pos(), but the label
  1907  		// declaration may be erroneous due to a token after that position (and
  1908  		// before the ':'). If SpuriousErrors is not set, the (only) error
  1909  		// reported for the line is the illegal label error instead of the token
  1910  		// before the ':' that caused the problem. Thus, use the (latest) colon
  1911  		// position for error reporting.
  1912  		p.error(colon, "illegal label declaration")
  1913  		return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false
  1914  
  1915  	case token.ARROW:
  1916  		// send statement
  1917  		arrow := p.pos
  1918  		p.next()
  1919  		y := p.parseRhs()
  1920  		return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false
  1921  
  1922  	case token.INC, token.DEC:
  1923  		// increment or decrement
  1924  		s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
  1925  		p.next()
  1926  		return s, false
  1927  	}
  1928  
  1929  	// expression
  1930  	return &ast.ExprStmt{X: x[0]}, false
  1931  }
  1932  
  1933  func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
  1934  	x := p.parseRhs() // could be a conversion: (some type)(x)
  1935  	if t := unparen(x); t != x {
  1936  		p.error(x.Pos(), fmt.Sprintf("expression in %s must not be parenthesized", callType))
  1937  		x = t
  1938  	}
  1939  	if call, isCall := x.(*ast.CallExpr); isCall {
  1940  		return call
  1941  	}
  1942  	if _, isBad := x.(*ast.BadExpr); !isBad {
  1943  		// only report error if it's a new one
  1944  		p.error(p.safePos(x.End()), fmt.Sprintf("expression in %s must be function call", callType))
  1945  	}
  1946  	return nil
  1947  }
  1948  
  1949  func (p *parser) parseGoStmt() ast.Stmt {
  1950  	if p.trace {
  1951  		defer un(trace(p, "GoStmt"))
  1952  	}
  1953  
  1954  	pos := p.expect(token.GO)
  1955  	call := p.parseCallExpr("go")
  1956  	p.expectSemi()
  1957  	if call == nil {
  1958  		return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
  1959  	}
  1960  
  1961  	return &ast.GoStmt{Go: pos, Call: call}
  1962  }
  1963  
  1964  func (p *parser) parseDeferStmt() ast.Stmt {
  1965  	if p.trace {
  1966  		defer un(trace(p, "DeferStmt"))
  1967  	}
  1968  
  1969  	pos := p.expect(token.DEFER)
  1970  	call := p.parseCallExpr("defer")
  1971  	p.expectSemi()
  1972  	if call == nil {
  1973  		return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
  1974  	}
  1975  
  1976  	return &ast.DeferStmt{Defer: pos, Call: call}
  1977  }
  1978  
  1979  func (p *parser) parseReturnStmt() *ast.ReturnStmt {
  1980  	if p.trace {
  1981  		defer un(trace(p, "ReturnStmt"))
  1982  	}
  1983  
  1984  	pos := p.pos
  1985  	p.expect(token.RETURN)
  1986  	var x []ast.Expr
  1987  	if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
  1988  		x = p.parseList(true)
  1989  	}
  1990  	p.expectSemi()
  1991  
  1992  	return &ast.ReturnStmt{Return: pos, Results: x}
  1993  }
  1994  
  1995  func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
  1996  	if p.trace {
  1997  		defer un(trace(p, "BranchStmt"))
  1998  	}
  1999  
  2000  	pos := p.expect(tok)
  2001  	var label *ast.Ident
  2002  	if tok != token.FALLTHROUGH && p.tok == token.IDENT {
  2003  		label = p.parseIdent()
  2004  	}
  2005  	p.expectSemi()
  2006  
  2007  	return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
  2008  }
  2009  
  2010  func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
  2011  	if s == nil {
  2012  		return nil
  2013  	}
  2014  	if es, isExpr := s.(*ast.ExprStmt); isExpr {
  2015  		return es.X
  2016  	}
  2017  	found := "simple statement"
  2018  	if _, isAss := s.(*ast.AssignStmt); isAss {
  2019  		found = "assignment"
  2020  	}
  2021  	p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
  2022  	return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
  2023  }
  2024  
  2025  // parseIfHeader is an adjusted version of parser.header
  2026  // in cmd/compile/internal/syntax/parser.go, which has
  2027  // been tuned for better error handling.
  2028  func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
  2029  	if p.tok == token.LBRACE {
  2030  		p.error(p.pos, "missing condition in if statement")
  2031  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2032  		return
  2033  	}
  2034  	// p.tok != token.LBRACE
  2035  
  2036  	prevLev := p.exprLev
  2037  	p.exprLev = -1
  2038  
  2039  	if p.tok != token.SEMICOLON {
  2040  		// accept potential variable declaration but complain
  2041  		if p.tok == token.VAR {
  2042  			p.next()
  2043  			p.error(p.pos, "var declaration not allowed in if initializer")
  2044  		}
  2045  		init, _ = p.parseSimpleStmt(basic)
  2046  	}
  2047  
  2048  	var condStmt ast.Stmt
  2049  	var semi struct {
  2050  		pos token.Pos
  2051  		lit string // ";" or "\n"; valid if pos.IsValid()
  2052  	}
  2053  	if p.tok != token.LBRACE {
  2054  		if p.tok == token.SEMICOLON {
  2055  			semi.pos = p.pos
  2056  			semi.lit = p.lit
  2057  			p.next()
  2058  		} else {
  2059  			p.expect(token.SEMICOLON)
  2060  		}
  2061  		if p.tok != token.LBRACE {
  2062  			condStmt, _ = p.parseSimpleStmt(basic)
  2063  		}
  2064  	} else {
  2065  		condStmt = init
  2066  		init = nil
  2067  	}
  2068  
  2069  	if condStmt != nil {
  2070  		cond = p.makeExpr(condStmt, "boolean expression")
  2071  	} else if semi.pos.IsValid() {
  2072  		if semi.lit == "\n" {
  2073  			p.error(semi.pos, "unexpected newline, expecting { after if clause")
  2074  		} else {
  2075  			p.error(semi.pos, "missing condition in if statement")
  2076  		}
  2077  	}
  2078  
  2079  	// make sure we have a valid AST
  2080  	if cond == nil {
  2081  		cond = &ast.BadExpr{From: p.pos, To: p.pos}
  2082  	}
  2083  
  2084  	p.exprLev = prevLev
  2085  	return
  2086  }
  2087  
  2088  func (p *parser) parseIfStmt() *ast.IfStmt {
  2089  	defer decNestLev(incNestLev(p))
  2090  
  2091  	if p.trace {
  2092  		defer un(trace(p, "IfStmt"))
  2093  	}
  2094  
  2095  	pos := p.expect(token.IF)
  2096  
  2097  	init, cond := p.parseIfHeader()
  2098  	body := p.parseBlockStmt()
  2099  
  2100  	var else_ ast.Stmt
  2101  	if p.tok == token.ELSE {
  2102  		p.next()
  2103  		switch p.tok {
  2104  		case token.IF:
  2105  			else_ = p.parseIfStmt()
  2106  		case token.LBRACE:
  2107  			else_ = p.parseBlockStmt()
  2108  			p.expectSemi()
  2109  		default:
  2110  			p.errorExpected(p.pos, "if statement or block")
  2111  			else_ = &ast.BadStmt{From: p.pos, To: p.pos}
  2112  		}
  2113  	} else {
  2114  		p.expectSemi()
  2115  	}
  2116  
  2117  	return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
  2118  }
  2119  
  2120  func (p *parser) parseCaseClause() *ast.CaseClause {
  2121  	if p.trace {
  2122  		defer un(trace(p, "CaseClause"))
  2123  	}
  2124  
  2125  	pos := p.pos
  2126  	var list []ast.Expr
  2127  	if p.tok == token.CASE {
  2128  		p.next()
  2129  		list = p.parseList(true)
  2130  	} else {
  2131  		p.expect(token.DEFAULT)
  2132  	}
  2133  
  2134  	colon := p.expect(token.COLON)
  2135  	body := p.parseStmtList()
  2136  
  2137  	return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
  2138  }
  2139  
  2140  func isTypeSwitchAssert(x ast.Expr) bool {
  2141  	a, ok := x.(*ast.TypeAssertExpr)
  2142  	return ok && a.Type == nil
  2143  }
  2144  
  2145  func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
  2146  	switch t := s.(type) {
  2147  	case *ast.ExprStmt:
  2148  		// x.(type)
  2149  		return isTypeSwitchAssert(t.X)
  2150  	case *ast.AssignStmt:
  2151  		// v := x.(type)
  2152  		if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
  2153  			switch t.Tok {
  2154  			case token.ASSIGN:
  2155  				// permit v = x.(type) but complain
  2156  				p.error(t.TokPos, "expected ':=', found '='")
  2157  				fallthrough
  2158  			case token.DEFINE:
  2159  				return true
  2160  			}
  2161  		}
  2162  	}
  2163  	return false
  2164  }
  2165  
  2166  func (p *parser) parseSwitchStmt() ast.Stmt {
  2167  	if p.trace {
  2168  		defer un(trace(p, "SwitchStmt"))
  2169  	}
  2170  
  2171  	pos := p.expect(token.SWITCH)
  2172  
  2173  	var s1, s2 ast.Stmt
  2174  	if p.tok != token.LBRACE {
  2175  		prevLev := p.exprLev
  2176  		p.exprLev = -1
  2177  		if p.tok != token.SEMICOLON {
  2178  			s2, _ = p.parseSimpleStmt(basic)
  2179  		}
  2180  		if p.tok == token.SEMICOLON {
  2181  			p.next()
  2182  			s1 = s2
  2183  			s2 = nil
  2184  			if p.tok != token.LBRACE {
  2185  				// A TypeSwitchGuard may declare a variable in addition
  2186  				// to the variable declared in the initial SimpleStmt.
  2187  				// Introduce extra scope to avoid redeclaration errors:
  2188  				//
  2189  				//	switch t := 0; t := x.(T) { ... }
  2190  				//
  2191  				// (this code is not valid Go because the first t
  2192  				// cannot be accessed and thus is never used, the extra
  2193  				// scope is needed for the correct error message).
  2194  				//
  2195  				// If we don't have a type switch, s2 must be an expression.
  2196  				// Having the extra nested but empty scope won't affect it.
  2197  				s2, _ = p.parseSimpleStmt(basic)
  2198  			}
  2199  		}
  2200  		p.exprLev = prevLev
  2201  	}
  2202  
  2203  	typeSwitch := p.isTypeSwitchGuard(s2)
  2204  	lbrace := p.expect(token.LBRACE)
  2205  	var list []ast.Stmt
  2206  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2207  		list = append(list, p.parseCaseClause())
  2208  	}
  2209  	rbrace := p.expect(token.RBRACE)
  2210  	p.expectSemi()
  2211  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2212  
  2213  	if typeSwitch {
  2214  		return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
  2215  	}
  2216  
  2217  	return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
  2218  }
  2219  
  2220  func (p *parser) parseCommClause() *ast.CommClause {
  2221  	if p.trace {
  2222  		defer un(trace(p, "CommClause"))
  2223  	}
  2224  
  2225  	pos := p.pos
  2226  	var comm ast.Stmt
  2227  	if p.tok == token.CASE {
  2228  		p.next()
  2229  		lhs := p.parseList(false)
  2230  		if p.tok == token.ARROW {
  2231  			// SendStmt
  2232  			if len(lhs) > 1 {
  2233  				p.errorExpected(lhs[0].Pos(), "1 expression")
  2234  				// continue with first expression
  2235  			}
  2236  			arrow := p.pos
  2237  			p.next()
  2238  			rhs := p.parseRhs()
  2239  			comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
  2240  		} else {
  2241  			// RecvStmt
  2242  			if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
  2243  				// RecvStmt with assignment
  2244  				if len(lhs) > 2 {
  2245  					p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
  2246  					// continue with first two expressions
  2247  					lhs = lhs[0:2]
  2248  				}
  2249  				pos := p.pos
  2250  				p.next()
  2251  				rhs := p.parseRhs()
  2252  				comm = &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
  2253  			} else {
  2254  				// lhs must be single receive operation
  2255  				if len(lhs) > 1 {
  2256  					p.errorExpected(lhs[0].Pos(), "1 expression")
  2257  					// continue with first expression
  2258  				}
  2259  				comm = &ast.ExprStmt{X: lhs[0]}
  2260  			}
  2261  		}
  2262  	} else {
  2263  		p.expect(token.DEFAULT)
  2264  	}
  2265  
  2266  	colon := p.expect(token.COLON)
  2267  	body := p.parseStmtList()
  2268  
  2269  	return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
  2270  }
  2271  
  2272  func (p *parser) parseSelectStmt() *ast.SelectStmt {
  2273  	if p.trace {
  2274  		defer un(trace(p, "SelectStmt"))
  2275  	}
  2276  
  2277  	pos := p.expect(token.SELECT)
  2278  	lbrace := p.expect(token.LBRACE)
  2279  	var list []ast.Stmt
  2280  	for p.tok == token.CASE || p.tok == token.DEFAULT {
  2281  		list = append(list, p.parseCommClause())
  2282  	}
  2283  	rbrace := p.expect(token.RBRACE)
  2284  	p.expectSemi()
  2285  	body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
  2286  
  2287  	return &ast.SelectStmt{Select: pos, Body: body}
  2288  }
  2289  
  2290  func (p *parser) parseForStmt() ast.Stmt {
  2291  	if p.trace {
  2292  		defer un(trace(p, "ForStmt"))
  2293  	}
  2294  
  2295  	pos := p.expect(token.FOR)
  2296  
  2297  	var s1, s2, s3 ast.Stmt
  2298  	var isRange bool
  2299  	if p.tok != token.LBRACE {
  2300  		prevLev := p.exprLev
  2301  		p.exprLev = -1
  2302  		if p.tok != token.SEMICOLON {
  2303  			if p.tok == token.RANGE {
  2304  				// "for range x" (nil lhs in assignment)
  2305  				pos := p.pos
  2306  				p.next()
  2307  				y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
  2308  				s2 = &ast.AssignStmt{Rhs: y}
  2309  				isRange = true
  2310  			} else {
  2311  				s2, isRange = p.parseSimpleStmt(rangeOk)
  2312  			}
  2313  		}
  2314  		if !isRange && p.tok == token.SEMICOLON {
  2315  			p.next()
  2316  			s1 = s2
  2317  			s2 = nil
  2318  			if p.tok != token.SEMICOLON {
  2319  				s2, _ = p.parseSimpleStmt(basic)
  2320  			}
  2321  			p.expectSemi()
  2322  			if p.tok != token.LBRACE {
  2323  				s3, _ = p.parseSimpleStmt(basic)
  2324  			}
  2325  		}
  2326  		p.exprLev = prevLev
  2327  	}
  2328  
  2329  	body := p.parseBlockStmt()
  2330  	p.expectSemi()
  2331  
  2332  	if isRange {
  2333  		as := s2.(*ast.AssignStmt)
  2334  		// check lhs
  2335  		var key, value ast.Expr
  2336  		switch len(as.Lhs) {
  2337  		case 0:
  2338  			// nothing to do
  2339  		case 1:
  2340  			key = as.Lhs[0]
  2341  		case 2:
  2342  			key, value = as.Lhs[0], as.Lhs[1]
  2343  		default:
  2344  			p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
  2345  			return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
  2346  		}
  2347  		// parseSimpleStmt returned a right-hand side that
  2348  		// is a single unary expression of the form "range x"
  2349  		x := as.Rhs[0].(*ast.UnaryExpr).X
  2350  		return &ast.RangeStmt{
  2351  			For:    pos,
  2352  			Key:    key,
  2353  			Value:  value,
  2354  			TokPos: as.TokPos,
  2355  			Tok:    as.Tok,
  2356  			Range:  as.Rhs[0].Pos(),
  2357  			X:      x,
  2358  			Body:   body,
  2359  		}
  2360  	}
  2361  
  2362  	// regular for statement
  2363  	return &ast.ForStmt{
  2364  		For:  pos,
  2365  		Init: s1,
  2366  		Cond: p.makeExpr(s2, "boolean or range expression"),
  2367  		Post: s3,
  2368  		Body: body,
  2369  	}
  2370  }
  2371  
  2372  func (p *parser) parseStmt() (s ast.Stmt) {
  2373  	defer decNestLev(incNestLev(p))
  2374  
  2375  	if p.trace {
  2376  		defer un(trace(p, "Statement"))
  2377  	}
  2378  
  2379  	switch p.tok {
  2380  	case token.CONST, token.TYPE, token.VAR:
  2381  		s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
  2382  	case
  2383  		// tokens that may start an expression
  2384  		token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
  2385  		token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
  2386  		token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
  2387  		s, _ = p.parseSimpleStmt(labelOk)
  2388  		// because of the required look-ahead, labeled statements are
  2389  		// parsed by parseSimpleStmt - don't expect a semicolon after
  2390  		// them
  2391  		if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
  2392  			p.expectSemi()
  2393  		}
  2394  	case token.GO:
  2395  		s = p.parseGoStmt()
  2396  	case token.DEFER:
  2397  		s = p.parseDeferStmt()
  2398  	case token.RETURN:
  2399  		s = p.parseReturnStmt()
  2400  	case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
  2401  		s = p.parseBranchStmt(p.tok)
  2402  	case token.LBRACE:
  2403  		s = p.parseBlockStmt()
  2404  		p.expectSemi()
  2405  	case token.IF:
  2406  		s = p.parseIfStmt()
  2407  	case token.SWITCH:
  2408  		s = p.parseSwitchStmt()
  2409  	case token.SELECT:
  2410  		s = p.parseSelectStmt()
  2411  	case token.FOR:
  2412  		s = p.parseForStmt()
  2413  	case token.SEMICOLON:
  2414  		// Is it ever possible to have an implicit semicolon
  2415  		// producing an empty statement in a valid program?
  2416  		// (handle correctly anyway)
  2417  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
  2418  		p.next()
  2419  	case token.RBRACE:
  2420  		// a semicolon may be omitted before a closing "}"
  2421  		s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
  2422  	default:
  2423  		// no statement found
  2424  		pos := p.pos
  2425  		p.errorExpected(pos, "statement")
  2426  		p.advance(stmtStart)
  2427  		s = &ast.BadStmt{From: pos, To: p.pos}
  2428  	}
  2429  
  2430  	return
  2431  }
  2432  
  2433  // ----------------------------------------------------------------------------
  2434  // Declarations
  2435  
  2436  type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec
  2437  
  2438  func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2439  	if p.trace {
  2440  		defer un(trace(p, "ImportSpec"))
  2441  	}
  2442  
  2443  	var ident *ast.Ident
  2444  	switch p.tok {
  2445  	case token.IDENT:
  2446  		ident = p.parseIdent()
  2447  	case token.PERIOD:
  2448  		ident = &ast.Ident{NamePos: p.pos, Name: "."}
  2449  		p.next()
  2450  	}
  2451  
  2452  	pos := p.pos
  2453  	var path string
  2454  	if p.tok == token.STRING {
  2455  		path = p.lit
  2456  		p.next()
  2457  	} else if p.tok.IsLiteral() {
  2458  		p.error(pos, "import path must be a string")
  2459  		p.next()
  2460  	} else {
  2461  		p.error(pos, "missing import path")
  2462  		p.advance(exprEnd)
  2463  	}
  2464  	comment := p.expectSemi()
  2465  
  2466  	// collect imports
  2467  	spec := &ast.ImportSpec{
  2468  		Doc:     doc,
  2469  		Name:    ident,
  2470  		Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
  2471  		Comment: comment,
  2472  	}
  2473  	p.imports = append(p.imports, spec)
  2474  
  2475  	return spec
  2476  }
  2477  
  2478  func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
  2479  	if p.trace {
  2480  		defer un(trace(p, keyword.String()+"Spec"))
  2481  	}
  2482  
  2483  	idents := p.parseIdentList()
  2484  	var typ ast.Expr
  2485  	var values []ast.Expr
  2486  	switch keyword {
  2487  	case token.CONST:
  2488  		// always permit optional type and initialization for more tolerant parsing
  2489  		if p.tok != token.EOF && p.tok != token.SEMICOLON && p.tok != token.RPAREN {
  2490  			typ = p.tryIdentOrType()
  2491  			if p.tok == token.ASSIGN {
  2492  				p.next()
  2493  				values = p.parseList(true)
  2494  			}
  2495  		}
  2496  	case token.VAR:
  2497  		if p.tok != token.ASSIGN {
  2498  			typ = p.parseType()
  2499  		}
  2500  		if p.tok == token.ASSIGN {
  2501  			p.next()
  2502  			values = p.parseList(true)
  2503  		}
  2504  	default:
  2505  		panic("unreachable")
  2506  	}
  2507  	comment := p.expectSemi()
  2508  
  2509  	spec := &ast.ValueSpec{
  2510  		Doc:     doc,
  2511  		Names:   idents,
  2512  		Type:    typ,
  2513  		Values:  values,
  2514  		Comment: comment,
  2515  	}
  2516  	return spec
  2517  }
  2518  
  2519  func (p *parser) parseGenericType(spec *ast.TypeSpec, openPos token.Pos, name0 *ast.Ident, typ0 ast.Expr) {
  2520  	if p.trace {
  2521  		defer un(trace(p, "parseGenericType"))
  2522  	}
  2523  
  2524  	list := p.parseParameterList(name0, typ0, token.RBRACK)
  2525  	closePos := p.expect(token.RBRACK)
  2526  	spec.TypeParams = &ast.FieldList{Opening: openPos, List: list, Closing: closePos}
  2527  	// Let the type checker decide whether to accept type parameters on aliases:
  2528  	// see issue #46477.
  2529  	if p.tok == token.ASSIGN {
  2530  		// type alias
  2531  		spec.Assign = p.pos
  2532  		p.next()
  2533  	}
  2534  	spec.Type = p.parseType()
  2535  }
  2536  
  2537  func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
  2538  	if p.trace {
  2539  		defer un(trace(p, "TypeSpec"))
  2540  	}
  2541  
  2542  	name := p.parseIdent()
  2543  	spec := &ast.TypeSpec{Doc: doc, Name: name}
  2544  
  2545  	if p.tok == token.LBRACK {
  2546  		// spec.Name "[" ...
  2547  		// array/slice type or type parameter list
  2548  		lbrack := p.pos
  2549  		p.next()
  2550  		if p.tok == token.IDENT {
  2551  			// We may have an array type or a type parameter list.
  2552  			// In either case we expect an expression x (which may
  2553  			// just be a name, or a more complex expression) which
  2554  			// we can analyze further.
  2555  			//
  2556  			// A type parameter list may have a type bound starting
  2557  			// with a "[" as in: P []E. In that case, simply parsing
  2558  			// an expression would lead to an error: P[] is invalid.
  2559  			// But since index or slice expressions are never constant
  2560  			// and thus invalid array length expressions, if the name
  2561  			// is followed by "[" it must be the start of an array or
  2562  			// slice constraint. Only if we don't see a "[" do we
  2563  			// need to parse a full expression. Notably, name <- x
  2564  			// is not a concern because name <- x is a statement and
  2565  			// not an expression.
  2566  			var x ast.Expr = p.parseIdent()
  2567  			if p.tok != token.LBRACK {
  2568  				// To parse the expression starting with name, expand
  2569  				// the call sequence we would get by passing in name
  2570  				// to parser.expr, and pass in name to parsePrimaryExpr.
  2571  				p.exprLev++
  2572  				lhs := p.parsePrimaryExpr(x)
  2573  				x = p.parseBinaryExpr(lhs, token.LowestPrec+1)
  2574  				p.exprLev--
  2575  			}
  2576  			// Analyze expression x. If we can split x into a type parameter
  2577  			// name, possibly followed by a type parameter type, we consider
  2578  			// this the start of a type parameter list, with some caveats:
  2579  			// a single name followed by "]" tilts the decision towards an
  2580  			// array declaration; a type parameter type that could also be
  2581  			// an ordinary expression but which is followed by a comma tilts
  2582  			// the decision towards a type parameter list.
  2583  			if pname, ptype := extractName(x, p.tok == token.COMMA); pname != nil && (ptype != nil || p.tok != token.RBRACK) {
  2584  				// spec.Name "[" pname ...
  2585  				// spec.Name "[" pname ptype ...
  2586  				// spec.Name "[" pname ptype "," ...
  2587  				p.parseGenericType(spec, lbrack, pname, ptype) // ptype may be nil
  2588  			} else {
  2589  				// spec.Name "[" pname "]" ...
  2590  				// spec.Name "[" x ...
  2591  				spec.Type = p.parseArrayType(lbrack, x)
  2592  			}
  2593  		} else {
  2594  			// array type
  2595  			spec.Type = p.parseArrayType(lbrack, nil)
  2596  		}
  2597  	} else {
  2598  		// no type parameters
  2599  		if p.tok == token.ASSIGN {
  2600  			// type alias
  2601  			spec.Assign = p.pos
  2602  			p.next()
  2603  		}
  2604  		spec.Type = p.parseType()
  2605  	}
  2606  
  2607  	spec.Comment = p.expectSemi()
  2608  
  2609  	return spec
  2610  }
  2611  
  2612  // extractName splits the expression x into (name, expr) if syntactically
  2613  // x can be written as name expr. The split only happens if expr is a type
  2614  // element (per the isTypeElem predicate) or if force is set.
  2615  // If x is just a name, the result is (name, nil). If the split succeeds,
  2616  // the result is (name, expr). Otherwise the result is (nil, x).
  2617  // Examples:
  2618  //
  2619  //	x           force    name    expr
  2620  //	------------------------------------
  2621  //	P*[]int     T/F      P       *[]int
  2622  //	P*E         T        P       *E
  2623  //	P*E         F        nil     P*E
  2624  //	P([]int)    T/F      P       []int
  2625  //	P(E)        T        P       E
  2626  //	P(E)        F        nil     P(E)
  2627  //	P*E|F|~G    T/F      P       *E|F|~G
  2628  //	P*E|F|G     T        P       *E|F|G
  2629  //	P*E|F|G     F        nil     P*E|F|G
  2630  func extractName(x ast.Expr, force bool) (*ast.Ident, ast.Expr) {
  2631  	switch x := x.(type) {
  2632  	case *ast.Ident:
  2633  		return x, nil
  2634  	case *ast.BinaryExpr:
  2635  		switch x.Op {
  2636  		case token.MUL:
  2637  			if name, _ := x.X.(*ast.Ident); name != nil && (force || isTypeElem(x.Y)) {
  2638  				// x = name *x.Y
  2639  				return name, &ast.StarExpr{Star: x.OpPos, X: x.Y}
  2640  			}
  2641  		case token.OR:
  2642  			if name, lhs := extractName(x.X, force || isTypeElem(x.Y)); name != nil && lhs != nil {
  2643  				// x = name lhs|x.Y
  2644  				op := *x
  2645  				op.X = lhs
  2646  				return name, &op
  2647  			}
  2648  		}
  2649  	case *ast.CallExpr:
  2650  		if name, _ := x.Fun.(*ast.Ident); name != nil {
  2651  			if len(x.Args) == 1 && x.Ellipsis == token.NoPos && (force || isTypeElem(x.Args[0])) {
  2652  				// x = name "(" x.ArgList[0] ")"
  2653  				return name, x.Args[0]
  2654  			}
  2655  		}
  2656  	}
  2657  	return nil, x
  2658  }
  2659  
  2660  // isTypeElem reports whether x is a (possibly parenthesized) type element expression.
  2661  // The result is false if x could be a type element OR an ordinary (value) expression.
  2662  func isTypeElem(x ast.Expr) bool {
  2663  	switch x := x.(type) {
  2664  	case *ast.ArrayType, *ast.StructType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.ChanType:
  2665  		return true
  2666  	case *ast.BinaryExpr:
  2667  		return isTypeElem(x.X) || isTypeElem(x.Y)
  2668  	case *ast.UnaryExpr:
  2669  		return x.Op == token.TILDE
  2670  	case *ast.ParenExpr:
  2671  		return isTypeElem(x.X)
  2672  	}
  2673  	return false
  2674  }
  2675  
  2676  func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
  2677  	if p.trace {
  2678  		defer un(trace(p, "GenDecl("+keyword.String()+")"))
  2679  	}
  2680  
  2681  	doc := p.leadComment
  2682  	pos := p.expect(keyword)
  2683  	var lparen, rparen token.Pos
  2684  	var list []ast.Spec
  2685  	if p.tok == token.LPAREN {
  2686  		lparen = p.pos
  2687  		p.next()
  2688  		for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
  2689  			list = append(list, f(p.leadComment, keyword, iota))
  2690  		}
  2691  		rparen = p.expect(token.RPAREN)
  2692  		p.expectSemi()
  2693  	} else {
  2694  		list = append(list, f(nil, keyword, 0))
  2695  	}
  2696  
  2697  	return &ast.GenDecl{
  2698  		Doc:    doc,
  2699  		TokPos: pos,
  2700  		Tok:    keyword,
  2701  		Lparen: lparen,
  2702  		Specs:  list,
  2703  		Rparen: rparen,
  2704  	}
  2705  }
  2706  
  2707  func (p *parser) parseFuncDecl() *ast.FuncDecl {
  2708  	if p.trace {
  2709  		defer un(trace(p, "FunctionDecl"))
  2710  	}
  2711  
  2712  	doc := p.leadComment
  2713  	pos := p.expect(token.FUNC)
  2714  
  2715  	var recv *ast.FieldList
  2716  	if p.tok == token.LPAREN {
  2717  		_, recv = p.parseParameters(false)
  2718  	}
  2719  
  2720  	ident := p.parseIdent()
  2721  
  2722  	tparams, params := p.parseParameters(true)
  2723  	if recv != nil && tparams != nil {
  2724  		// Method declarations do not have type parameters. We parse them for a
  2725  		// better error message and improved error recovery.
  2726  		p.error(tparams.Opening, "method must have no type parameters")
  2727  		tparams = nil
  2728  	}
  2729  	results := p.parseResult()
  2730  
  2731  	var body *ast.BlockStmt
  2732  	switch p.tok {
  2733  	case token.LBRACE:
  2734  		body = p.parseBody()
  2735  		p.expectSemi()
  2736  	case token.SEMICOLON:
  2737  		p.next()
  2738  		if p.tok == token.LBRACE {
  2739  			// opening { of function declaration on next line
  2740  			p.error(p.pos, "unexpected semicolon or newline before {")
  2741  			body = p.parseBody()
  2742  			p.expectSemi()
  2743  		}
  2744  	default:
  2745  		p.expectSemi()
  2746  	}
  2747  
  2748  	decl := &ast.FuncDecl{
  2749  		Doc:  doc,
  2750  		Recv: recv,
  2751  		Name: ident,
  2752  		Type: &ast.FuncType{
  2753  			Func:       pos,
  2754  			TypeParams: tparams,
  2755  			Params:     params,
  2756  			Results:    results,
  2757  		},
  2758  		Body: body,
  2759  	}
  2760  	return decl
  2761  }
  2762  
  2763  func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
  2764  	if p.trace {
  2765  		defer un(trace(p, "Declaration"))
  2766  	}
  2767  
  2768  	var f parseSpecFunction
  2769  	switch p.tok {
  2770  	case token.IMPORT:
  2771  		f = p.parseImportSpec
  2772  
  2773  	case token.CONST, token.VAR:
  2774  		f = p.parseValueSpec
  2775  
  2776  	case token.TYPE:
  2777  		f = p.parseTypeSpec
  2778  
  2779  	case token.FUNC:
  2780  		return p.parseFuncDecl()
  2781  
  2782  	default:
  2783  		pos := p.pos
  2784  		p.errorExpected(pos, "declaration")
  2785  		p.advance(sync)
  2786  		return &ast.BadDecl{From: pos, To: p.pos}
  2787  	}
  2788  
  2789  	return p.parseGenDecl(p.tok, f)
  2790  }
  2791  
  2792  // ----------------------------------------------------------------------------
  2793  // Source files
  2794  
  2795  func (p *parser) parseFile() *ast.File {
  2796  	if p.trace {
  2797  		defer un(trace(p, "File"))
  2798  	}
  2799  
  2800  	// Don't bother parsing the rest if we had errors scanning the first token.
  2801  	// Likely not a Go source file at all.
  2802  	if p.errors.Len() != 0 {
  2803  		return nil
  2804  	}
  2805  
  2806  	// package clause
  2807  	doc := p.leadComment
  2808  	pos := p.expect(token.PACKAGE)
  2809  	// Go spec: The package clause is not a declaration;
  2810  	// the package name does not appear in any scope.
  2811  	ident := p.parseIdent()
  2812  	if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
  2813  		p.error(p.pos, "invalid package name _")
  2814  	}
  2815  	p.expectSemi()
  2816  
  2817  	// Don't bother parsing the rest if we had errors parsing the package clause.
  2818  	// Likely not a Go source file at all.
  2819  	if p.errors.Len() != 0 {
  2820  		return nil
  2821  	}
  2822  
  2823  	var decls []ast.Decl
  2824  	if p.mode&PackageClauseOnly == 0 {
  2825  		// import decls
  2826  		for p.tok == token.IMPORT {
  2827  			decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
  2828  		}
  2829  
  2830  		if p.mode&ImportsOnly == 0 {
  2831  			// rest of package body
  2832  			prev := token.IMPORT
  2833  			for p.tok != token.EOF {
  2834  				// Continue to accept import declarations for error tolerance, but complain.
  2835  				if p.tok == token.IMPORT && prev != token.IMPORT {
  2836  					p.error(p.pos, "imports must appear before other declarations")
  2837  				}
  2838  				prev = p.tok
  2839  
  2840  				decls = append(decls, p.parseDecl(declStart))
  2841  			}
  2842  		}
  2843  	}
  2844  
  2845  	f := &ast.File{
  2846  		Doc:       doc,
  2847  		Package:   pos,
  2848  		Name:      ident,
  2849  		Decls:     decls,
  2850  		FileStart: token.Pos(p.file.Base()),
  2851  		FileEnd:   token.Pos(p.file.Base() + p.file.Size()),
  2852  		Imports:   p.imports,
  2853  		Comments:  p.comments,
  2854  	}
  2855  	var declErr func(token.Pos, string)
  2856  	if p.mode&DeclarationErrors != 0 {
  2857  		declErr = p.error
  2858  	}
  2859  	if p.mode&SkipObjectResolution == 0 {
  2860  		resolveFile(f, p.file, declErr)
  2861  	}
  2862  
  2863  	return f
  2864  }
  2865  

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