call.go

     1  // Copyright 2013 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  // This file implements typechecking of call and selector expressions.
     6  
     7  package types
     8  
     9  import (
    10  	"go/ast"
    11  	"go/token"
    12  	. "internal/types/errors"
    13  	"strings"
    14  )
    15  
    16  // funcInst type-checks a function instantiation.
    17  // The incoming x must be a generic function.
    18  // If ix != nil, it provides some or all of the type arguments (ix.Indices).
    19  // If target != nil, it may be used to infer missing type arguments of x, if any.
    20  // At least one of T or ix must be provided.
    21  //
    22  // There are two modes of operation:
    23  //
    24  //  1. If infer == true, funcInst infers missing type arguments as needed and
    25  //     instantiates the function x. The returned results are nil.
    26  //
    27  //  2. If infer == false and inst provides all type arguments, funcInst
    28  //     instantiates the function x. The returned results are nil.
    29  //     If inst doesn't provide enough type arguments, funcInst returns the
    30  //     available arguments and the corresponding expression list; x remains
    31  //     unchanged.
    32  //
    33  // If an error (other than a version error) occurs in any case, it is reported
    34  // and x.mode is set to invalid.
    35  func (check *Checker) funcInst(T *target, pos token.Pos, x *operand, ix *indexedExpr, infer bool) ([]Type, []ast.Expr) {
    36  	assert(T != nil || ix != nil)
    37  
    38  	var instErrPos positioner
    39  	if ix != nil {
    40  		instErrPos = inNode(ix.orig, ix.lbrack)
    41  		x.expr = ix.orig // if we don't have an index expression, keep the existing expression of x
    42  	} else {
    43  		instErrPos = atPos(pos)
    44  	}
    45  	versionErr := !check.verifyVersionf(instErrPos, go1_18, "function instantiation")
    46  
    47  	// targs and xlist are the type arguments and corresponding type expressions, or nil.
    48  	var targs []Type
    49  	var xlist []ast.Expr
    50  	if ix != nil {
    51  		xlist = ix.indices
    52  		targs = check.typeList(xlist)
    53  		if targs == nil {
    54  			x.mode = invalid
    55  			return nil, nil
    56  		}
    57  		assert(len(targs) == len(xlist))
    58  	}
    59  
    60  	// Check the number of type arguments (got) vs number of type parameters (want).
    61  	// Note that x is a function value, not a type expression, so we don't need to
    62  	// call under below.
    63  	sig := x.typ.(*Signature)
    64  	got, want := len(targs), sig.TypeParams().Len()
    65  	if got > want {
    66  		// Providing too many type arguments is always an error.
    67  		check.errorf(ix.indices[got-1], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
    68  		x.mode = invalid
    69  		return nil, nil
    70  	}
    71  
    72  	if got < want {
    73  		if !infer {
    74  			return targs, xlist
    75  		}
    76  
    77  		// If the uninstantiated or partially instantiated function x is used in
    78  		// an assignment (tsig != nil), infer missing type arguments by treating
    79  		// the assignment
    80  		//
    81  		//    var tvar tsig = x
    82  		//
    83  		// like a call g(tvar) of the synthetic generic function g
    84  		//
    85  		//    func g[type_parameters_of_x](func_type_of_x)
    86  		//
    87  		var args []*operand
    88  		var params []*Var
    89  		var reverse bool
    90  		if T != nil && sig.tparams != nil {
    91  			if !versionErr && !check.allowVersion(go1_21) {
    92  				if ix != nil {
    93  					check.versionErrorf(instErrPos, go1_21, "partially instantiated function in assignment")
    94  				} else {
    95  					check.versionErrorf(instErrPos, go1_21, "implicitly instantiated function in assignment")
    96  				}
    97  			}
    98  			gsig := NewSignatureType(nil, nil, nil, sig.params, sig.results, sig.variadic)
    99  			params = []*Var{NewVar(x.Pos(), check.pkg, "", gsig)}
   100  			// The type of the argument operand is tsig, which is the type of the LHS in an assignment
   101  			// or the result type in a return statement. Create a pseudo-expression for that operand
   102  			// that makes sense when reported in error messages from infer, below.
   103  			expr := ast.NewIdent(T.desc)
   104  			expr.NamePos = x.Pos() // correct position
   105  			args = []*operand{{mode: value, expr: expr, typ: T.sig}}
   106  			reverse = true
   107  		}
   108  
   109  		// Rename type parameters to avoid problems with recursive instantiations.
   110  		// Note that NewTuple(params...) below is (*Tuple)(nil) if len(params) == 0, as desired.
   111  		tparams, params2 := check.renameTParams(pos, sig.TypeParams().list(), NewTuple(params...))
   112  
   113  		err := check.newError(CannotInferTypeArgs)
   114  		targs = check.infer(atPos(pos), tparams, targs, params2.(*Tuple), args, reverse, err)
   115  		if targs == nil {
   116  			if !err.empty() {
   117  				err.report()
   118  			}
   119  			x.mode = invalid
   120  			return nil, nil
   121  		}
   122  		got = len(targs)
   123  	}
   124  	assert(got == want)
   125  
   126  	// instantiate function signature
   127  	sig = check.instantiateSignature(x.Pos(), x.expr, sig, targs, xlist)
   128  	x.typ = sig
   129  	x.mode = value
   130  	return nil, nil
   131  }
   132  
   133  func (check *Checker) instantiateSignature(pos token.Pos, expr ast.Expr, typ *Signature, targs []Type, xlist []ast.Expr) (res *Signature) {
   134  	assert(check != nil)
   135  	assert(len(targs) == typ.TypeParams().Len())
   136  
   137  	if check.conf._Trace {
   138  		check.trace(pos, "-- instantiating signature %s with %s", typ, targs)
   139  		check.indent++
   140  		defer func() {
   141  			check.indent--
   142  			check.trace(pos, "=> %s (under = %s)", res, res.Underlying())
   143  		}()
   144  	}
   145  
   146  	// For signatures, Checker.instance will always succeed because the type argument
   147  	// count is correct at this point (see assertion above); hence the type assertion
   148  	// to *Signature will always succeed.
   149  	inst := check.instance(pos, typ, targs, nil, check.context()).(*Signature)
   150  	assert(inst.TypeParams().Len() == 0) // signature is not generic anymore
   151  	check.recordInstance(expr, targs, inst)
   152  	assert(len(xlist) <= len(targs))
   153  
   154  	// verify instantiation lazily (was go.dev/issue/50450)
   155  	check.later(func() {
   156  		tparams := typ.TypeParams().list()
   157  		// check type constraints
   158  		if i, err := check.verify(pos, tparams, targs, check.context()); err != nil {
   159  			// best position for error reporting
   160  			pos := pos
   161  			if i < len(xlist) {
   162  				pos = xlist[i].Pos()
   163  			}
   164  			check.softErrorf(atPos(pos), InvalidTypeArg, "%s", err)
   165  		} else {
   166  			check.mono.recordInstance(check.pkg, pos, tparams, targs, xlist)
   167  		}
   168  	}).describef(atPos(pos), "verify instantiation")
   169  
   170  	return inst
   171  }
   172  
   173  func (check *Checker) callExpr(x *operand, call *ast.CallExpr) exprKind {
   174  	ix := unpackIndexedExpr(call.Fun)
   175  	if ix != nil {
   176  		if check.indexExpr(x, ix) {
   177  			// Delay function instantiation to argument checking,
   178  			// where we combine type and value arguments for type
   179  			// inference.
   180  			assert(x.mode == value)
   181  		} else {
   182  			ix = nil
   183  		}
   184  		x.expr = call.Fun
   185  		check.record(x)
   186  	} else {
   187  		check.exprOrType(x, call.Fun, true)
   188  	}
   189  	// x.typ may be generic
   190  
   191  	switch x.mode {
   192  	case invalid:
   193  		check.use(call.Args...)
   194  		x.expr = call
   195  		return statement
   196  
   197  	case typexpr:
   198  		// conversion
   199  		check.nonGeneric(nil, x)
   200  		if x.mode == invalid {
   201  			return conversion
   202  		}
   203  		T := x.typ
   204  		x.mode = invalid
   205  		switch n := len(call.Args); n {
   206  		case 0:
   207  			check.errorf(inNode(call, call.Rparen), WrongArgCount, "missing argument in conversion to %s", T)
   208  		case 1:
   209  			check.expr(nil, x, call.Args[0])
   210  			if x.mode != invalid {
   211  				if hasDots(call) {
   212  					check.errorf(call.Args[0], BadDotDotDotSyntax, "invalid use of ... in conversion to %s", T)
   213  					break
   214  				}
   215  				if t, _ := under(T).(*Interface); t != nil && !isTypeParam(T) {
   216  					if !t.IsMethodSet() {
   217  						check.errorf(call, MisplacedConstraintIface, "cannot use interface %s in conversion (contains specific type constraints or is comparable)", T)
   218  						break
   219  					}
   220  				}
   221  				check.conversion(x, T)
   222  			}
   223  		default:
   224  			check.use(call.Args...)
   225  			check.errorf(call.Args[n-1], WrongArgCount, "too many arguments in conversion to %s", T)
   226  		}
   227  		x.expr = call
   228  		return conversion
   229  
   230  	case builtin:
   231  		// no need to check for non-genericity here
   232  		id := x.id
   233  		if !check.builtin(x, call, id) {
   234  			x.mode = invalid
   235  		}
   236  		x.expr = call
   237  		// a non-constant result implies a function call
   238  		if x.mode != invalid && x.mode != constant_ {
   239  			check.hasCallOrRecv = true
   240  		}
   241  		return predeclaredFuncs[id].kind
   242  	}
   243  
   244  	// ordinary function/method call
   245  	// signature may be generic
   246  	cgocall := x.mode == cgofunc
   247  
   248  	// a type parameter may be "called" if all types have the same signature
   249  	sig, _ := coreType(x.typ).(*Signature)
   250  	if sig == nil {
   251  		check.errorf(x, InvalidCall, invalidOp+"cannot call non-function %s", x)
   252  		x.mode = invalid
   253  		x.expr = call
   254  		return statement
   255  	}
   256  
   257  	// Capture wasGeneric before sig is potentially instantiated below.
   258  	wasGeneric := sig.TypeParams().Len() > 0
   259  
   260  	// evaluate type arguments, if any
   261  	var xlist []ast.Expr
   262  	var targs []Type
   263  	if ix != nil {
   264  		xlist = ix.indices
   265  		targs = check.typeList(xlist)
   266  		if targs == nil {
   267  			check.use(call.Args...)
   268  			x.mode = invalid
   269  			x.expr = call
   270  			return statement
   271  		}
   272  		assert(len(targs) == len(xlist))
   273  
   274  		// check number of type arguments (got) vs number of type parameters (want)
   275  		got, want := len(targs), sig.TypeParams().Len()
   276  		if got > want {
   277  			check.errorf(xlist[want], WrongTypeArgCount, "got %d type arguments but want %d", got, want)
   278  			check.use(call.Args...)
   279  			x.mode = invalid
   280  			x.expr = call
   281  			return statement
   282  		}
   283  
   284  		// If sig is generic and all type arguments are provided, preempt function
   285  		// argument type inference by explicitly instantiating the signature. This
   286  		// ensures that we record accurate type information for sig, even if there
   287  		// is an error checking its arguments (for example, if an incorrect number
   288  		// of arguments is supplied).
   289  		if got == want && want > 0 {
   290  			check.verifyVersionf(atPos(ix.lbrack), go1_18, "function instantiation")
   291  			sig = check.instantiateSignature(ix.Pos(), ix.orig, sig, targs, xlist)
   292  			// targs have been consumed; proceed with checking arguments of the
   293  			// non-generic signature.
   294  			targs = nil
   295  			xlist = nil
   296  		}
   297  	}
   298  
   299  	// evaluate arguments
   300  	args, atargs, atxlist := check.genericExprList(call.Args)
   301  	sig = check.arguments(call, sig, targs, xlist, args, atargs, atxlist)
   302  
   303  	if wasGeneric && sig.TypeParams().Len() == 0 {
   304  		// Update the recorded type of call.Fun to its instantiated type.
   305  		check.recordTypeAndValue(call.Fun, value, sig, nil)
   306  	}
   307  
   308  	// determine result
   309  	switch sig.results.Len() {
   310  	case 0:
   311  		x.mode = novalue
   312  	case 1:
   313  		if cgocall {
   314  			x.mode = commaerr
   315  		} else {
   316  			x.mode = value
   317  		}
   318  		x.typ = sig.results.vars[0].typ // unpack tuple
   319  	default:
   320  		x.mode = value
   321  		x.typ = sig.results
   322  	}
   323  	x.expr = call
   324  	check.hasCallOrRecv = true
   325  
   326  	// if type inference failed, a parameterized result must be invalidated
   327  	// (operands cannot have a parameterized type)
   328  	if x.mode == value && sig.TypeParams().Len() > 0 && isParameterized(sig.TypeParams().list(), x.typ) {
   329  		x.mode = invalid
   330  	}
   331  
   332  	return statement
   333  }
   334  
   335  // exprList evaluates a list of expressions and returns the corresponding operands.
   336  // A single-element expression list may evaluate to multiple operands.
   337  func (check *Checker) exprList(elist []ast.Expr) (xlist []*operand) {
   338  	if n := len(elist); n == 1 {
   339  		xlist, _ = check.multiExpr(elist[0], false)
   340  	} else if n > 1 {
   341  		// multiple (possibly invalid) values
   342  		xlist = make([]*operand, n)
   343  		for i, e := range elist {
   344  			var x operand
   345  			check.expr(nil, &x, e)
   346  			xlist[i] = &x
   347  		}
   348  	}
   349  	return
   350  }
   351  
   352  // genericExprList is like exprList but result operands may be uninstantiated or partially
   353  // instantiated generic functions (where constraint information is insufficient to infer
   354  // the missing type arguments) for Go 1.21 and later.
   355  // For each non-generic or uninstantiated generic operand, the corresponding targsList and
   356  // xlistList elements do not exist (targsList and xlistList are nil) or the elements are nil.
   357  // For each partially instantiated generic function operand, the corresponding targsList and
   358  // xlistList elements are the operand's partial type arguments and type expression lists.
   359  func (check *Checker) genericExprList(elist []ast.Expr) (resList []*operand, targsList [][]Type, xlistList [][]ast.Expr) {
   360  	if debug {
   361  		defer func() {
   362  			// targsList and xlistList must have matching lengths
   363  			assert(len(targsList) == len(xlistList))
   364  			// type arguments must only exist for partially instantiated functions
   365  			for i, x := range resList {
   366  				if i < len(targsList) {
   367  					if n := len(targsList[i]); n > 0 {
   368  						// x must be a partially instantiated function
   369  						assert(n < x.typ.(*Signature).TypeParams().Len())
   370  					}
   371  				}
   372  			}
   373  		}()
   374  	}
   375  
   376  	// Before Go 1.21, uninstantiated or partially instantiated argument functions are
   377  	// nor permitted. Checker.funcInst must infer missing type arguments in that case.
   378  	infer := true // for -lang < go1.21
   379  	n := len(elist)
   380  	if n > 0 && check.allowVersion(go1_21) {
   381  		infer = false
   382  	}
   383  
   384  	if n == 1 {
   385  		// single value (possibly a partially instantiated function), or a multi-valued expression
   386  		e := elist[0]
   387  		var x operand
   388  		if ix := unpackIndexedExpr(e); ix != nil && check.indexExpr(&x, ix) {
   389  			// x is a generic function.
   390  			targs, xlist := check.funcInst(nil, x.Pos(), &x, ix, infer)
   391  			if targs != nil {
   392  				// x was not instantiated: collect the (partial) type arguments.
   393  				targsList = [][]Type{targs}
   394  				xlistList = [][]ast.Expr{xlist}
   395  				// Update x.expr so that we can record the partially instantiated function.
   396  				x.expr = ix.orig
   397  			} else {
   398  				// x was instantiated: we must record it here because we didn't
   399  				// use the usual expression evaluators.
   400  				check.record(&x)
   401  			}
   402  			resList = []*operand{&x}
   403  		} else {
   404  			// x is not a function instantiation (it may still be a generic function).
   405  			check.rawExpr(nil, &x, e, nil, true)
   406  			check.exclude(&x, 1<<novalue|1<<builtin|1<<typexpr)
   407  			if t, ok := x.typ.(*Tuple); ok && x.mode != invalid {
   408  				// x is a function call returning multiple values; it cannot be generic.
   409  				resList = make([]*operand, t.Len())
   410  				for i, v := range t.vars {
   411  					resList[i] = &operand{mode: value, expr: e, typ: v.typ}
   412  				}
   413  			} else {
   414  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   415  				resList = []*operand{&x}
   416  			}
   417  		}
   418  	} else if n > 1 {
   419  		// multiple values
   420  		resList = make([]*operand, n)
   421  		targsList = make([][]Type, n)
   422  		xlistList = make([][]ast.Expr, n)
   423  		for i, e := range elist {
   424  			var x operand
   425  			if ix := unpackIndexedExpr(e); ix != nil && check.indexExpr(&x, ix) {
   426  				// x is a generic function.
   427  				targs, xlist := check.funcInst(nil, x.Pos(), &x, ix, infer)
   428  				if targs != nil {
   429  					// x was not instantiated: collect the (partial) type arguments.
   430  					targsList[i] = targs
   431  					xlistList[i] = xlist
   432  					// Update x.expr so that we can record the partially instantiated function.
   433  					x.expr = ix.orig
   434  				} else {
   435  					// x was instantiated: we must record it here because we didn't
   436  					// use the usual expression evaluators.
   437  					check.record(&x)
   438  				}
   439  			} else {
   440  				// x is exactly one value (possibly invalid or uninstantiated generic function).
   441  				check.genericExpr(&x, e)
   442  			}
   443  			resList[i] = &x
   444  		}
   445  	}
   446  
   447  	return
   448  }
   449  
   450  // arguments type-checks arguments passed to a function call with the given signature.
   451  // The function and its arguments may be generic, and possibly partially instantiated.
   452  // targs and xlist are the function's type arguments (and corresponding expressions).
   453  // args are the function arguments. If an argument args[i] is a partially instantiated
   454  // generic function, atargs[i] and atxlist[i] are the corresponding type arguments
   455  // (and corresponding expressions).
   456  // If the callee is variadic, arguments adjusts its signature to match the provided
   457  // arguments. The type parameters and arguments of the callee and all its arguments
   458  // are used together to infer any missing type arguments, and the callee and argument
   459  // functions are instantiated as necessary.
   460  // The result signature is the (possibly adjusted and instantiated) function signature.
   461  // If an error occurred, the result signature is the incoming sig.
   462  func (check *Checker) arguments(call *ast.CallExpr, sig *Signature, targs []Type, xlist []ast.Expr, args []*operand, atargs [][]Type, atxlist [][]ast.Expr) (rsig *Signature) {
   463  	rsig = sig
   464  
   465  	// Function call argument/parameter count requirements
   466  	//
   467  	//               | standard call    | dotdotdot call |
   468  	// --------------+------------------+----------------+
   469  	// standard func | nargs == npars   | invalid        |
   470  	// --------------+------------------+----------------+
   471  	// variadic func | nargs >= npars-1 | nargs == npars |
   472  	// --------------+------------------+----------------+
   473  
   474  	nargs := len(args)
   475  	npars := sig.params.Len()
   476  	ddd := hasDots(call)
   477  
   478  	// set up parameters
   479  	sigParams := sig.params // adjusted for variadic functions (may be nil for empty parameter lists!)
   480  	adjusted := false       // indicates if sigParams is different from sig.params
   481  	if sig.variadic {
   482  		if ddd {
   483  			// variadic_func(a, b, c...)
   484  			if len(call.Args) == 1 && nargs > 1 {
   485  				// f()... is not permitted if f() is multi-valued
   486  				check.errorf(inNode(call, call.Ellipsis), InvalidDotDotDot, "cannot use ... with %d-valued %s", nargs, call.Args[0])
   487  				return
   488  			}
   489  		} else {
   490  			// variadic_func(a, b, c)
   491  			if nargs >= npars-1 {
   492  				// Create custom parameters for arguments: keep
   493  				// the first npars-1 parameters and add one for
   494  				// each argument mapping to the ... parameter.
   495  				vars := make([]*Var, npars-1) // npars > 0 for variadic functions
   496  				copy(vars, sig.params.vars)
   497  				last := sig.params.vars[npars-1]
   498  				typ := last.typ.(*Slice).elem
   499  				for len(vars) < nargs {
   500  					vars = append(vars, NewParam(last.pos, last.pkg, last.name, typ))
   501  				}
   502  				sigParams = NewTuple(vars...) // possibly nil!
   503  				adjusted = true
   504  				npars = nargs
   505  			} else {
   506  				// nargs < npars-1
   507  				npars-- // for correct error message below
   508  			}
   509  		}
   510  	} else {
   511  		if ddd {
   512  			// standard_func(a, b, c...)
   513  			check.errorf(inNode(call, call.Ellipsis), NonVariadicDotDotDot, "cannot use ... in call to non-variadic %s", call.Fun)
   514  			return
   515  		}
   516  		// standard_func(a, b, c)
   517  	}
   518  
   519  	// check argument count
   520  	if nargs != npars {
   521  		var at positioner = call
   522  		qualifier := "not enough"
   523  		if nargs > npars {
   524  			at = args[npars].expr // report at first extra argument
   525  			qualifier = "too many"
   526  		} else {
   527  			at = atPos(call.Rparen) // report at closing )
   528  		}
   529  		// take care of empty parameter lists represented by nil tuples
   530  		var params []*Var
   531  		if sig.params != nil {
   532  			params = sig.params.vars
   533  		}
   534  		err := check.newError(WrongArgCount)
   535  		err.addf(at, "%s arguments in call to %s", qualifier, call.Fun)
   536  		err.addf(noposn, "have %s", check.typesSummary(operandTypes(args), false, ddd))
   537  		err.addf(noposn, "want %s", check.typesSummary(varTypes(params), sig.variadic, false))
   538  		err.report()
   539  		return
   540  	}
   541  
   542  	// collect type parameters of callee and generic function arguments
   543  	var tparams []*TypeParam
   544  
   545  	// collect type parameters of callee
   546  	n := sig.TypeParams().Len()
   547  	if n > 0 {
   548  		if !check.allowVersion(go1_18) {
   549  			switch call.Fun.(type) {
   550  			case *ast.IndexExpr, *ast.IndexListExpr:
   551  				ix := unpackIndexedExpr(call.Fun)
   552  				check.versionErrorf(inNode(call.Fun, ix.lbrack), go1_18, "function instantiation")
   553  			default:
   554  				check.versionErrorf(inNode(call, call.Lparen), go1_18, "implicit function instantiation")
   555  			}
   556  		}
   557  		// rename type parameters to avoid problems with recursive calls
   558  		var tmp Type
   559  		tparams, tmp = check.renameTParams(call.Pos(), sig.TypeParams().list(), sigParams)
   560  		sigParams = tmp.(*Tuple)
   561  		// make sure targs and tparams have the same length
   562  		for len(targs) < len(tparams) {
   563  			targs = append(targs, nil)
   564  		}
   565  	}
   566  	assert(len(tparams) == len(targs))
   567  
   568  	// collect type parameters from generic function arguments
   569  	var genericArgs []int // indices of generic function arguments
   570  	if enableReverseTypeInference {
   571  		for i, arg := range args {
   572  			// generic arguments cannot have a defined (*Named) type - no need for underlying type below
   573  			if asig, _ := arg.typ.(*Signature); asig != nil && asig.TypeParams().Len() > 0 {
   574  				// The argument type is a generic function signature. This type is
   575  				// pointer-identical with (it's copied from) the type of the generic
   576  				// function argument and thus the function object.
   577  				// Before we change the type (type parameter renaming, below), make
   578  				// a clone of it as otherwise we implicitly modify the object's type
   579  				// (go.dev/issues/63260).
   580  				asig = clone(asig)
   581  				// Rename type parameters for cases like f(g, g); this gives each
   582  				// generic function argument a unique type identity (go.dev/issues/59956).
   583  				// TODO(gri) Consider only doing this if a function argument appears
   584  				//           multiple times, which is rare (possible optimization).
   585  				atparams, tmp := check.renameTParams(call.Pos(), asig.TypeParams().list(), asig)
   586  				asig = tmp.(*Signature)
   587  				asig.tparams = &TypeParamList{atparams} // renameTParams doesn't touch associated type parameters
   588  				arg.typ = asig                          // new type identity for the function argument
   589  				tparams = append(tparams, atparams...)
   590  				// add partial list of type arguments, if any
   591  				if i < len(atargs) {
   592  					targs = append(targs, atargs[i]...)
   593  				}
   594  				// make sure targs and tparams have the same length
   595  				for len(targs) < len(tparams) {
   596  					targs = append(targs, nil)
   597  				}
   598  				genericArgs = append(genericArgs, i)
   599  			}
   600  		}
   601  	}
   602  	assert(len(tparams) == len(targs))
   603  
   604  	// at the moment we only support implicit instantiations of argument functions
   605  	_ = len(genericArgs) > 0 && check.verifyVersionf(args[genericArgs[0]], go1_21, "implicitly instantiated function as argument")
   606  
   607  	// tparams holds the type parameters of the callee and generic function arguments, if any:
   608  	// the first n type parameters belong to the callee, followed by mi type parameters for each
   609  	// of the generic function arguments, where mi = args[i].typ.(*Signature).TypeParams().Len().
   610  
   611  	// infer missing type arguments of callee and function arguments
   612  	if len(tparams) > 0 {
   613  		err := check.newError(CannotInferTypeArgs)
   614  		targs = check.infer(call, tparams, targs, sigParams, args, false, err)
   615  		if targs == nil {
   616  			// TODO(gri) If infer inferred the first targs[:n], consider instantiating
   617  			//           the call signature for better error messages/gopls behavior.
   618  			//           Perhaps instantiate as much as we can, also for arguments.
   619  			//           This will require changes to how infer returns its results.
   620  			if !err.empty() {
   621  				check.errorf(err.posn(), CannotInferTypeArgs, "in call to %s, %s", call.Fun, err.msg())
   622  			}
   623  			return
   624  		}
   625  
   626  		// update result signature: instantiate if needed
   627  		if n > 0 {
   628  			rsig = check.instantiateSignature(call.Pos(), call.Fun, sig, targs[:n], xlist)
   629  			// If the callee's parameter list was adjusted we need to update (instantiate)
   630  			// it separately. Otherwise we can simply use the result signature's parameter
   631  			// list.
   632  			if adjusted {
   633  				sigParams = check.subst(call.Pos(), sigParams, makeSubstMap(tparams[:n], targs[:n]), nil, check.context()).(*Tuple)
   634  			} else {
   635  				sigParams = rsig.params
   636  			}
   637  		}
   638  
   639  		// compute argument signatures: instantiate if needed
   640  		j := n
   641  		for _, i := range genericArgs {
   642  			arg := args[i]
   643  			asig := arg.typ.(*Signature)
   644  			k := j + asig.TypeParams().Len()
   645  			// targs[j:k] are the inferred type arguments for asig
   646  			arg.typ = check.instantiateSignature(call.Pos(), arg.expr, asig, targs[j:k], nil) // TODO(gri) provide xlist if possible (partial instantiations)
   647  			check.record(arg)                                                                 // record here because we didn't use the usual expr evaluators
   648  			j = k
   649  		}
   650  	}
   651  
   652  	// check arguments
   653  	if len(args) > 0 {
   654  		context := check.sprintf("argument to %s", call.Fun)
   655  		for i, a := range args {
   656  			check.assignment(a, sigParams.vars[i].typ, context)
   657  		}
   658  	}
   659  
   660  	return
   661  }
   662  
   663  var cgoPrefixes = [...]string{
   664  	"_Ciconst_",
   665  	"_Cfconst_",
   666  	"_Csconst_",
   667  	"_Ctype_",
   668  	"_Cvar_", // actually a pointer to the var
   669  	"_Cfpvar_fp_",
   670  	"_Cfunc_",
   671  	"_Cmacro_", // function to evaluate the expanded expression
   672  }
   673  
   674  func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *TypeName, wantType bool) {
   675  	// these must be declared before the "goto Error" statements
   676  	var (
   677  		obj      Object
   678  		index    []int
   679  		indirect bool
   680  	)
   681  
   682  	sel := e.Sel.Name
   683  	// If the identifier refers to a package, handle everything here
   684  	// so we don't need a "package" mode for operands: package names
   685  	// can only appear in qualified identifiers which are mapped to
   686  	// selector expressions.
   687  	if ident, ok := e.X.(*ast.Ident); ok {
   688  		obj := check.lookup(ident.Name)
   689  		if pname, _ := obj.(*PkgName); pname != nil {
   690  			assert(pname.pkg == check.pkg)
   691  			check.recordUse(ident, pname)
   692  			pname.used = true
   693  			pkg := pname.imported
   694  
   695  			var exp Object
   696  			funcMode := value
   697  			if pkg.cgo {
   698  				// cgo special cases C.malloc: it's
   699  				// rewritten to _CMalloc and does not
   700  				// support two-result calls.
   701  				if sel == "malloc" {
   702  					sel = "_CMalloc"
   703  				} else {
   704  					funcMode = cgofunc
   705  				}
   706  				for _, prefix := range cgoPrefixes {
   707  					// cgo objects are part of the current package (in file
   708  					// _cgo_gotypes.go). Use regular lookup.
   709  					exp = check.lookup(prefix + sel)
   710  					if exp != nil {
   711  						break
   712  					}
   713  				}
   714  				if exp == nil {
   715  					if isValidName(sel) {
   716  						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
   717  					}
   718  					goto Error
   719  				}
   720  				check.objDecl(exp, nil)
   721  			} else {
   722  				exp = pkg.scope.Lookup(sel)
   723  				if exp == nil {
   724  					if !pkg.fake && isValidName(sel) {
   725  						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
   726  					}
   727  					goto Error
   728  				}
   729  				if !exp.Exported() {
   730  					check.errorf(e.Sel, UnexportedName, "name %s not exported by package %s", sel, pkg.name)
   731  					// ok to continue
   732  				}
   733  			}
   734  			check.recordUse(e.Sel, exp)
   735  
   736  			// Simplified version of the code for *ast.Idents:
   737  			// - imported objects are always fully initialized
   738  			switch exp := exp.(type) {
   739  			case *Const:
   740  				assert(exp.Val() != nil)
   741  				x.mode = constant_
   742  				x.typ = exp.typ
   743  				x.val = exp.val
   744  			case *TypeName:
   745  				x.mode = typexpr
   746  				x.typ = exp.typ
   747  			case *Var:
   748  				x.mode = variable
   749  				x.typ = exp.typ
   750  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cvar_") {
   751  					x.typ = x.typ.(*Pointer).base
   752  				}
   753  			case *Func:
   754  				x.mode = funcMode
   755  				x.typ = exp.typ
   756  				if pkg.cgo && strings.HasPrefix(exp.name, "_Cmacro_") {
   757  					x.mode = value
   758  					x.typ = x.typ.(*Signature).results.vars[0].typ
   759  				}
   760  			case *Builtin:
   761  				x.mode = builtin
   762  				x.typ = exp.typ
   763  				x.id = exp.id
   764  			default:
   765  				check.dump("%v: unexpected object %v", e.Sel.Pos(), exp)
   766  				panic("unreachable")
   767  			}
   768  			x.expr = e
   769  			return
   770  		}
   771  	}
   772  
   773  	check.exprOrType(x, e.X, false)
   774  	switch x.mode {
   775  	case typexpr:
   776  		// don't crash for "type T T.x" (was go.dev/issue/51509)
   777  		if def != nil && def.typ == x.typ {
   778  			check.cycleError([]Object{def}, 0)
   779  			goto Error
   780  		}
   781  	case builtin:
   782  		// types2 uses the position of '.' for the error
   783  		check.errorf(e.Sel, UncalledBuiltin, "invalid use of %s in selector expression", x)
   784  		goto Error
   785  	case invalid:
   786  		goto Error
   787  	}
   788  
   789  	// Avoid crashing when checking an invalid selector in a method declaration
   790  	// (i.e., where def is not set):
   791  	//
   792  	//   type S[T any] struct{}
   793  	//   type V = S[any]
   794  	//   func (fs *S[T]) M(x V.M) {}
   795  	//
   796  	// All codepaths below return a non-type expression. If we get here while
   797  	// expecting a type expression, it is an error.
   798  	//
   799  	// See go.dev/issue/57522 for more details.
   800  	//
   801  	// TODO(rfindley): We should do better by refusing to check selectors in all cases where
   802  	// x.typ is incomplete.
   803  	if wantType {
   804  		check.errorf(e.Sel, NotAType, "%s is not a type", ast.Expr(e))
   805  		goto Error
   806  	}
   807  
   808  	obj, index, indirect = lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel, false)
   809  	if obj == nil {
   810  		// Don't report another error if the underlying type was invalid (go.dev/issue/49541).
   811  		if !isValid(under(x.typ)) {
   812  			goto Error
   813  		}
   814  
   815  		if index != nil {
   816  			// TODO(gri) should provide actual type where the conflict happens
   817  			check.errorf(e.Sel, AmbiguousSelector, "ambiguous selector %s.%s", x.expr, sel)
   818  			goto Error
   819  		}
   820  
   821  		if indirect {
   822  			if x.mode == typexpr {
   823  				check.errorf(e.Sel, InvalidMethodExpr, "invalid method expression %s.%s (needs pointer receiver (*%s).%s)", x.typ, sel, x.typ, sel)
   824  			} else {
   825  				check.errorf(e.Sel, InvalidMethodExpr, "cannot call pointer method %s on %s", sel, x.typ)
   826  			}
   827  			goto Error
   828  		}
   829  
   830  		var why string
   831  		if isInterfacePtr(x.typ) {
   832  			why = check.interfacePtrError(x.typ)
   833  		} else {
   834  			alt, _, _ := lookupFieldOrMethod(x.typ, x.mode == variable, check.pkg, sel, true)
   835  			why = check.lookupError(x.typ, sel, alt, false)
   836  		}
   837  		check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (%s)", x.expr, sel, why)
   838  		goto Error
   839  	}
   840  
   841  	// methods may not have a fully set up signature yet
   842  	if m, _ := obj.(*Func); m != nil {
   843  		check.objDecl(m, nil)
   844  	}
   845  
   846  	if x.mode == typexpr {
   847  		// method expression
   848  		m, _ := obj.(*Func)
   849  		if m == nil {
   850  			check.errorf(e.Sel, MissingFieldOrMethod, "%s.%s undefined (type %s has no method %s)", x.expr, sel, x.typ, sel)
   851  			goto Error
   852  		}
   853  
   854  		check.recordSelection(e, MethodExpr, x.typ, m, index, indirect)
   855  
   856  		sig := m.typ.(*Signature)
   857  		if sig.recv == nil {
   858  			check.error(e, InvalidDeclCycle, "illegal cycle in method declaration")
   859  			goto Error
   860  		}
   861  
   862  		// the receiver type becomes the type of the first function
   863  		// argument of the method expression's function type
   864  		var params []*Var
   865  		if sig.params != nil {
   866  			params = sig.params.vars
   867  		}
   868  		// Be consistent about named/unnamed parameters. This is not needed
   869  		// for type-checking, but the newly constructed signature may appear
   870  		// in an error message and then have mixed named/unnamed parameters.
   871  		// (An alternative would be to not print parameter names in errors,
   872  		// but it's useful to see them; this is cheap and method expressions
   873  		// are rare.)
   874  		name := ""
   875  		if len(params) > 0 && params[0].name != "" {
   876  			// name needed
   877  			name = sig.recv.name
   878  			if name == "" {
   879  				name = "_"
   880  			}
   881  		}
   882  		params = append([]*Var{NewVar(sig.recv.pos, sig.recv.pkg, name, x.typ)}, params...)
   883  		x.mode = value
   884  		x.typ = &Signature{
   885  			tparams:  sig.tparams,
   886  			params:   NewTuple(params...),
   887  			results:  sig.results,
   888  			variadic: sig.variadic,
   889  		}
   890  
   891  		check.addDeclDep(m)
   892  
   893  	} else {
   894  		// regular selector
   895  		switch obj := obj.(type) {
   896  		case *Var:
   897  			check.recordSelection(e, FieldVal, x.typ, obj, index, indirect)
   898  			if x.mode == variable || indirect {
   899  				x.mode = variable
   900  			} else {
   901  				x.mode = value
   902  			}
   903  			x.typ = obj.typ
   904  
   905  		case *Func:
   906  			// TODO(gri) If we needed to take into account the receiver's
   907  			// addressability, should we report the type &(x.typ) instead?
   908  			check.recordSelection(e, MethodVal, x.typ, obj, index, indirect)
   909  
   910  			// TODO(gri) The verification pass below is disabled for now because
   911  			//           method sets don't match method lookup in some cases.
   912  			//           For instance, if we made a copy above when creating a
   913  			//           custom method for a parameterized received type, the
   914  			//           method set method doesn't match (no copy there). There
   915  			///          may be other situations.
   916  			disabled := true
   917  			if !disabled && debug {
   918  				// Verify that LookupFieldOrMethod and MethodSet.Lookup agree.
   919  				// TODO(gri) This only works because we call LookupFieldOrMethod
   920  				// _before_ calling NewMethodSet: LookupFieldOrMethod completes
   921  				// any incomplete interfaces so they are available to NewMethodSet
   922  				// (which assumes that interfaces have been completed already).
   923  				typ := x.typ
   924  				if x.mode == variable {
   925  					// If typ is not an (unnamed) pointer or an interface,
   926  					// use *typ instead, because the method set of *typ
   927  					// includes the methods of typ.
   928  					// Variables are addressable, so we can always take their
   929  					// address.
   930  					if _, ok := typ.(*Pointer); !ok && !IsInterface(typ) {
   931  						typ = &Pointer{base: typ}
   932  					}
   933  				}
   934  				// If we created a synthetic pointer type above, we will throw
   935  				// away the method set computed here after use.
   936  				// TODO(gri) Method set computation should probably always compute
   937  				// both, the value and the pointer receiver method set and represent
   938  				// them in a single structure.
   939  				// TODO(gri) Consider also using a method set cache for the lifetime
   940  				// of checker once we rely on MethodSet lookup instead of individual
   941  				// lookup.
   942  				mset := NewMethodSet(typ)
   943  				if m := mset.Lookup(check.pkg, sel); m == nil || m.obj != obj {
   944  					check.dump("%v: (%s).%v -> %s", e.Pos(), typ, obj.name, m)
   945  					check.dump("%s\n", mset)
   946  					// Caution: MethodSets are supposed to be used externally
   947  					// only (after all interface types were completed). It's
   948  					// now possible that we get here incorrectly. Not urgent
   949  					// to fix since we only run this code in debug mode.
   950  					// TODO(gri) fix this eventually.
   951  					panic("method sets and lookup don't agree")
   952  				}
   953  			}
   954  
   955  			x.mode = value
   956  
   957  			// remove receiver
   958  			sig := *obj.typ.(*Signature)
   959  			sig.recv = nil
   960  			x.typ = &sig
   961  
   962  			check.addDeclDep(obj)
   963  
   964  		default:
   965  			panic("unreachable")
   966  		}
   967  	}
   968  
   969  	// everything went well
   970  	x.expr = e
   971  	return
   972  
   973  Error:
   974  	x.mode = invalid
   975  	x.expr = e
   976  }
   977  
   978  // use type-checks each argument.
   979  // Useful to make sure expressions are evaluated
   980  // (and variables are "used") in the presence of
   981  // other errors. Arguments may be nil.
   982  // Reports if all arguments evaluated without error.
   983  func (check *Checker) use(args ...ast.Expr) bool { return check.useN(args, false) }
   984  
   985  // useLHS is like use, but doesn't "use" top-level identifiers.
   986  // It should be called instead of use if the arguments are
   987  // expressions on the lhs of an assignment.
   988  func (check *Checker) useLHS(args ...ast.Expr) bool { return check.useN(args, true) }
   989  
   990  func (check *Checker) useN(args []ast.Expr, lhs bool) bool {
   991  	ok := true
   992  	for _, e := range args {
   993  		if !check.use1(e, lhs) {
   994  			ok = false
   995  		}
   996  	}
   997  	return ok
   998  }
   999  
  1000  func (check *Checker) use1(e ast.Expr, lhs bool) bool {
  1001  	var x operand
  1002  	x.mode = value // anything but invalid
  1003  	switch n := ast.Unparen(e).(type) {
  1004  	case nil:
  1005  		// nothing to do
  1006  	case *ast.Ident:
  1007  		// don't report an error evaluating blank
  1008  		if n.Name == "_" {
  1009  			break
  1010  		}
  1011  		// If the lhs is an identifier denoting a variable v, this assignment
  1012  		// is not a 'use' of v. Remember current value of v.used and restore
  1013  		// after evaluating the lhs via check.rawExpr.
  1014  		var v *Var
  1015  		var v_used bool
  1016  		if lhs {
  1017  			if obj := check.lookup(n.Name); obj != nil {
  1018  				// It's ok to mark non-local variables, but ignore variables
  1019  				// from other packages to avoid potential race conditions with
  1020  				// dot-imported variables.
  1021  				if w, _ := obj.(*Var); w != nil && w.pkg == check.pkg {
  1022  					v = w
  1023  					v_used = v.used
  1024  				}
  1025  			}
  1026  		}
  1027  		check.exprOrType(&x, n, true)
  1028  		if v != nil {
  1029  			v.used = v_used // restore v.used
  1030  		}
  1031  	default:
  1032  		check.rawExpr(nil, &x, e, nil, true)
  1033  	}
  1034  	return x.mode != invalid
  1035  }
  1036
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