Source file src/go/types/named.go

     1  // Code generated by "go test -run=Generate -write=all"; DO NOT EDIT.
     2  
     3  // Copyright 2011 The Go Authors. All rights reserved.
     4  // Use of this source code is governed by a BSD-style
     5  // license that can be found in the LICENSE file.
     6  
     7  package types
     8  
     9  import (
    10  	"go/token"
    11  	"sync"
    12  	"sync/atomic"
    13  )
    14  
    15  // Type-checking Named types is subtle, because they may be recursively
    16  // defined, and because their full details may be spread across multiple
    17  // declarations (via methods). For this reason they are type-checked lazily,
    18  // to avoid information being accessed before it is complete.
    19  //
    20  // Conceptually, it is helpful to think of named types as having two distinct
    21  // sets of information:
    22  //  - "LHS" information, defining their identity: Obj() and TypeArgs()
    23  //  - "RHS" information, defining their details: TypeParams(), Underlying(),
    24  //    and methods.
    25  //
    26  // In this taxonomy, LHS information is available immediately, but RHS
    27  // information is lazy. Specifically, a named type N may be constructed in any
    28  // of the following ways:
    29  //  1. type-checked from the source
    30  //  2. loaded eagerly from export data
    31  //  3. loaded lazily from export data (when using unified IR)
    32  //  4. instantiated from a generic type
    33  //
    34  // In cases 1, 3, and 4, it is possible that the underlying type or methods of
    35  // N may not be immediately available.
    36  //  - During type-checking, we allocate N before type-checking its underlying
    37  //    type or methods, so that we may resolve recursive references.
    38  //  - When loading from export data, we may load its methods and underlying
    39  //    type lazily using a provided load function.
    40  //  - After instantiating, we lazily expand the underlying type and methods
    41  //    (note that instances may be created while still in the process of
    42  //    type-checking the original type declaration).
    43  //
    44  // In cases 3 and 4 this lazy construction may also occur concurrently, due to
    45  // concurrent use of the type checker API (after type checking or importing has
    46  // finished). It is critical that we keep track of state, so that Named types
    47  // are constructed exactly once and so that we do not access their details too
    48  // soon.
    49  //
    50  // We achieve this by tracking state with an atomic state variable, and
    51  // guarding potentially concurrent calculations with a mutex. At any point in
    52  // time this state variable determines which data on N may be accessed. As
    53  // state monotonically progresses, any data available at state M may be
    54  // accessed without acquiring the mutex at state N, provided N >= M.
    55  //
    56  // GLOSSARY: Here are a few terms used in this file to describe Named types:
    57  //  - We say that a Named type is "instantiated" if it has been constructed by
    58  //    instantiating a generic named type with type arguments.
    59  //  - We say that a Named type is "declared" if it corresponds to a type
    60  //    declaration in the source. Instantiated named types correspond to a type
    61  //    instantiation in the source, not a declaration. But their Origin type is
    62  //    a declared type.
    63  //  - We say that a Named type is "resolved" if its RHS information has been
    64  //    loaded or fully type-checked. For Named types constructed from export
    65  //    data, this may involve invoking a loader function to extract information
    66  //    from export data. For instantiated named types this involves reading
    67  //    information from their origin.
    68  //  - We say that a Named type is "expanded" if it is an instantiated type and
    69  //    type parameters in its underlying type and methods have been substituted
    70  //    with the type arguments from the instantiation. A type may be partially
    71  //    expanded if some but not all of these details have been substituted.
    72  //    Similarly, we refer to these individual details (underlying type or
    73  //    method) as being "expanded".
    74  //  - When all information is known for a named type, we say it is "complete".
    75  //
    76  // Some invariants to keep in mind: each declared Named type has a single
    77  // corresponding object, and that object's type is the (possibly generic) Named
    78  // type. Declared Named types are identical if and only if their pointers are
    79  // identical. On the other hand, multiple instantiated Named types may be
    80  // identical even though their pointers are not identical. One has to use
    81  // Identical to compare them. For instantiated named types, their obj is a
    82  // synthetic placeholder that records their position of the corresponding
    83  // instantiation in the source (if they were constructed during type checking).
    84  //
    85  // To prevent infinite expansion of named instances that are created outside of
    86  // type-checking, instances share a Context with other instances created during
    87  // their expansion. Via the pidgeonhole principle, this guarantees that in the
    88  // presence of a cycle of named types, expansion will eventually find an
    89  // existing instance in the Context and short-circuit the expansion.
    90  //
    91  // Once an instance is complete, we can nil out this shared Context to unpin
    92  // memory, though this Context may still be held by other incomplete instances
    93  // in its "lineage".
    94  
    95  // A Named represents a named (defined) type.
    96  type Named struct {
    97  	check *Checker  // non-nil during type-checking; nil otherwise
    98  	obj   *TypeName // corresponding declared object for declared types; see above for instantiated types
    99  
   100  	// fromRHS holds the type (on RHS of declaration) this *Named type is derived
   101  	// from (for cycle reporting). Only used by validType, and therefore does not
   102  	// require synchronization.
   103  	fromRHS Type
   104  
   105  	// information for instantiated types; nil otherwise
   106  	inst *instance
   107  
   108  	mu         sync.Mutex     // guards all fields below
   109  	state_     uint32         // the current state of this type; must only be accessed atomically
   110  	underlying Type           // possibly a *Named during setup; never a *Named once set up completely
   111  	tparams    *TypeParamList // type parameters, or nil
   112  
   113  	// methods declared for this type (not the method set of this type)
   114  	// Signatures are type-checked lazily.
   115  	// For non-instantiated types, this is a fully populated list of methods. For
   116  	// instantiated types, methods are individually expanded when they are first
   117  	// accessed.
   118  	methods []*Func
   119  
   120  	// loader may be provided to lazily load type parameters, underlying type, and methods.
   121  	loader func(*Named) (tparams []*TypeParam, underlying Type, methods []*Func)
   122  }
   123  
   124  // instance holds information that is only necessary for instantiated named
   125  // types.
   126  type instance struct {
   127  	orig            *Named    // original, uninstantiated type
   128  	targs           *TypeList // type arguments
   129  	expandedMethods int       // number of expanded methods; expandedMethods <= len(orig.methods)
   130  	ctxt            *Context  // local Context; set to nil after full expansion
   131  }
   132  
   133  // namedState represents the possible states that a named type may assume.
   134  type namedState uint32
   135  
   136  const (
   137  	unresolved namedState = iota // tparams, underlying type and methods might be unavailable
   138  	resolved                     // resolve has run; methods might be incomplete (for instances)
   139  	complete                     // all data is known
   140  )
   141  
   142  // NewNamed returns a new named type for the given type name, underlying type, and associated methods.
   143  // If the given type name obj doesn't have a type yet, its type is set to the returned named type.
   144  // The underlying type must not be a *Named.
   145  func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
   146  	if asNamed(underlying) != nil {
   147  		panic("underlying type must not be *Named")
   148  	}
   149  	return (*Checker)(nil).newNamed(obj, underlying, methods)
   150  }
   151  
   152  // resolve resolves the type parameters, methods, and underlying type of n.
   153  // This information may be loaded from a provided loader function, or computed
   154  // from an origin type (in the case of instances).
   155  //
   156  // After resolution, the type parameters, methods, and underlying type of n are
   157  // accessible; but if n is an instantiated type, its methods may still be
   158  // unexpanded.
   159  func (n *Named) resolve() *Named {
   160  	if n.state() >= resolved { // avoid locking below
   161  		return n
   162  	}
   163  
   164  	// TODO(rfindley): if n.check is non-nil we can avoid locking here, since
   165  	// type-checking is not concurrent. Evaluate if this is worth doing.
   166  	n.mu.Lock()
   167  	defer n.mu.Unlock()
   168  
   169  	if n.state() >= resolved {
   170  		return n
   171  	}
   172  
   173  	if n.inst != nil {
   174  		assert(n.underlying == nil) // n is an unresolved instance
   175  		assert(n.loader == nil)     // instances are created by instantiation, in which case n.loader is nil
   176  
   177  		orig := n.inst.orig
   178  		orig.resolve()
   179  		underlying := n.expandUnderlying()
   180  
   181  		n.tparams = orig.tparams
   182  		n.underlying = underlying
   183  		n.fromRHS = orig.fromRHS // for cycle detection
   184  
   185  		if len(orig.methods) == 0 {
   186  			n.setState(complete) // nothing further to do
   187  			n.inst.ctxt = nil
   188  		} else {
   189  			n.setState(resolved)
   190  		}
   191  		return n
   192  	}
   193  
   194  	// TODO(mdempsky): Since we're passing n to the loader anyway
   195  	// (necessary because types2 expects the receiver type for methods
   196  	// on defined interface types to be the Named rather than the
   197  	// underlying Interface), maybe it should just handle calling
   198  	// SetTypeParams, SetUnderlying, and AddMethod instead?  Those
   199  	// methods would need to support reentrant calls though. It would
   200  	// also make the API more future-proof towards further extensions.
   201  	if n.loader != nil {
   202  		assert(n.underlying == nil)
   203  		assert(n.TypeArgs().Len() == 0) // instances are created by instantiation, in which case n.loader is nil
   204  
   205  		tparams, underlying, methods := n.loader(n)
   206  
   207  		n.tparams = bindTParams(tparams)
   208  		n.underlying = underlying
   209  		n.fromRHS = underlying // for cycle detection
   210  		n.methods = methods
   211  		n.loader = nil
   212  	}
   213  
   214  	n.setState(complete)
   215  	return n
   216  }
   217  
   218  // state atomically accesses the current state of the receiver.
   219  func (n *Named) state() namedState {
   220  	return namedState(atomic.LoadUint32(&n.state_))
   221  }
   222  
   223  // setState atomically stores the given state for n.
   224  // Must only be called while holding n.mu.
   225  func (n *Named) setState(state namedState) {
   226  	atomic.StoreUint32(&n.state_, uint32(state))
   227  }
   228  
   229  // newNamed is like NewNamed but with a *Checker receiver.
   230  func (check *Checker) newNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
   231  	typ := &Named{check: check, obj: obj, fromRHS: underlying, underlying: underlying, methods: methods}
   232  	if obj.typ == nil {
   233  		obj.typ = typ
   234  	}
   235  	// Ensure that typ is always sanity-checked.
   236  	if check != nil {
   237  		check.needsCleanup(typ)
   238  	}
   239  	return typ
   240  }
   241  
   242  // newNamedInstance creates a new named instance for the given origin and type
   243  // arguments, recording pos as the position of its synthetic object (for error
   244  // reporting).
   245  //
   246  // If set, expanding is the named type instance currently being expanded, that
   247  // led to the creation of this instance.
   248  func (check *Checker) newNamedInstance(pos token.Pos, orig *Named, targs []Type, expanding *Named) *Named {
   249  	assert(len(targs) > 0)
   250  
   251  	obj := NewTypeName(pos, orig.obj.pkg, orig.obj.name, nil)
   252  	inst := &instance{orig: orig, targs: newTypeList(targs)}
   253  
   254  	// Only pass the expanding context to the new instance if their packages
   255  	// match. Since type reference cycles are only possible within a single
   256  	// package, this is sufficient for the purposes of short-circuiting cycles.
   257  	// Avoiding passing the context in other cases prevents unnecessary coupling
   258  	// of types across packages.
   259  	if expanding != nil && expanding.Obj().pkg == obj.pkg {
   260  		inst.ctxt = expanding.inst.ctxt
   261  	}
   262  	typ := &Named{check: check, obj: obj, inst: inst}
   263  	obj.typ = typ
   264  	// Ensure that typ is always sanity-checked.
   265  	if check != nil {
   266  		check.needsCleanup(typ)
   267  	}
   268  	return typ
   269  }
   270  
   271  func (t *Named) cleanup() {
   272  	assert(t.inst == nil || t.inst.orig.inst == nil)
   273  	// Ensure that every defined type created in the course of type-checking has
   274  	// either non-*Named underlying type, or is unexpanded.
   275  	//
   276  	// This guarantees that we don't leak any types whose underlying type is
   277  	// *Named, because any unexpanded instances will lazily compute their
   278  	// underlying type by substituting in the underlying type of their origin.
   279  	// The origin must have either been imported or type-checked and expanded
   280  	// here, and in either case its underlying type will be fully expanded.
   281  	switch t.underlying.(type) {
   282  	case nil:
   283  		if t.TypeArgs().Len() == 0 {
   284  			panic("nil underlying")
   285  		}
   286  	case *Named:
   287  		t.under() // t.under may add entries to check.cleaners
   288  	}
   289  	t.check = nil
   290  }
   291  
   292  // Obj returns the type name for the declaration defining the named type t. For
   293  // instantiated types, this is same as the type name of the origin type.
   294  func (t *Named) Obj() *TypeName {
   295  	if t.inst == nil {
   296  		return t.obj
   297  	}
   298  	return t.inst.orig.obj
   299  }
   300  
   301  // Origin returns the generic type from which the named type t is
   302  // instantiated. If t is not an instantiated type, the result is t.
   303  func (t *Named) Origin() *Named {
   304  	if t.inst == nil {
   305  		return t
   306  	}
   307  	return t.inst.orig
   308  }
   309  
   310  // TypeParams returns the type parameters of the named type t, or nil.
   311  // The result is non-nil for an (originally) generic type even if it is instantiated.
   312  func (t *Named) TypeParams() *TypeParamList { return t.resolve().tparams }
   313  
   314  // SetTypeParams sets the type parameters of the named type t.
   315  // t must not have type arguments.
   316  func (t *Named) SetTypeParams(tparams []*TypeParam) {
   317  	assert(t.inst == nil)
   318  	t.resolve().tparams = bindTParams(tparams)
   319  }
   320  
   321  // TypeArgs returns the type arguments used to instantiate the named type t.
   322  func (t *Named) TypeArgs() *TypeList {
   323  	if t.inst == nil {
   324  		return nil
   325  	}
   326  	return t.inst.targs
   327  }
   328  
   329  // NumMethods returns the number of explicit methods defined for t.
   330  func (t *Named) NumMethods() int {
   331  	return len(t.Origin().resolve().methods)
   332  }
   333  
   334  // Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
   335  //
   336  // For an ordinary or instantiated type t, the receiver base type of this
   337  // method is the named type t. For an uninstantiated generic type t, each
   338  // method receiver is instantiated with its receiver type parameters.
   339  func (t *Named) Method(i int) *Func {
   340  	t.resolve()
   341  
   342  	if t.state() >= complete {
   343  		return t.methods[i]
   344  	}
   345  
   346  	assert(t.inst != nil) // only instances should have incomplete methods
   347  	orig := t.inst.orig
   348  
   349  	t.mu.Lock()
   350  	defer t.mu.Unlock()
   351  
   352  	if len(t.methods) != len(orig.methods) {
   353  		assert(len(t.methods) == 0)
   354  		t.methods = make([]*Func, len(orig.methods))
   355  	}
   356  
   357  	if t.methods[i] == nil {
   358  		assert(t.inst.ctxt != nil) // we should still have a context remaining from the resolution phase
   359  		t.methods[i] = t.expandMethod(i)
   360  		t.inst.expandedMethods++
   361  
   362  		// Check if we've created all methods at this point. If we have, mark the
   363  		// type as fully expanded.
   364  		if t.inst.expandedMethods == len(orig.methods) {
   365  			t.setState(complete)
   366  			t.inst.ctxt = nil // no need for a context anymore
   367  		}
   368  	}
   369  
   370  	return t.methods[i]
   371  }
   372  
   373  // expandMethod substitutes type arguments in the i'th method for an
   374  // instantiated receiver.
   375  func (t *Named) expandMethod(i int) *Func {
   376  	// t.orig.methods is not lazy. origm is the method instantiated with its
   377  	// receiver type parameters (the "origin" method).
   378  	origm := t.inst.orig.Method(i)
   379  	assert(origm != nil)
   380  
   381  	check := t.check
   382  	// Ensure that the original method is type-checked.
   383  	if check != nil {
   384  		check.objDecl(origm, nil)
   385  	}
   386  
   387  	origSig := origm.typ.(*Signature)
   388  	rbase, _ := deref(origSig.Recv().Type())
   389  
   390  	// If rbase is t, then origm is already the instantiated method we're looking
   391  	// for. In this case, we return origm to preserve the invariant that
   392  	// traversing Method->Receiver Type->Method should get back to the same
   393  	// method.
   394  	//
   395  	// This occurs if t is instantiated with the receiver type parameters, as in
   396  	// the use of m in func (r T[_]) m() { r.m() }.
   397  	if rbase == t {
   398  		return origm
   399  	}
   400  
   401  	sig := origSig
   402  	// We can only substitute if we have a correspondence between type arguments
   403  	// and type parameters. This check is necessary in the presence of invalid
   404  	// code.
   405  	if origSig.RecvTypeParams().Len() == t.inst.targs.Len() {
   406  		smap := makeSubstMap(origSig.RecvTypeParams().list(), t.inst.targs.list())
   407  		var ctxt *Context
   408  		if check != nil {
   409  			ctxt = check.context()
   410  		}
   411  		sig = check.subst(origm.pos, origSig, smap, t, ctxt).(*Signature)
   412  	}
   413  
   414  	if sig == origSig {
   415  		// No substitution occurred, but we still need to create a new signature to
   416  		// hold the instantiated receiver.
   417  		copy := *origSig
   418  		sig = &copy
   419  	}
   420  
   421  	var rtyp Type
   422  	if origm.hasPtrRecv() {
   423  		rtyp = NewPointer(t)
   424  	} else {
   425  		rtyp = t
   426  	}
   427  
   428  	sig.recv = substVar(origSig.recv, rtyp)
   429  	return substFunc(origm, sig)
   430  }
   431  
   432  // SetUnderlying sets the underlying type and marks t as complete.
   433  // t must not have type arguments.
   434  func (t *Named) SetUnderlying(underlying Type) {
   435  	assert(t.inst == nil)
   436  	if underlying == nil {
   437  		panic("underlying type must not be nil")
   438  	}
   439  	if asNamed(underlying) != nil {
   440  		panic("underlying type must not be *Named")
   441  	}
   442  	t.resolve().underlying = underlying
   443  	if t.fromRHS == nil {
   444  		t.fromRHS = underlying // for cycle detection
   445  	}
   446  }
   447  
   448  // AddMethod adds method m unless it is already in the method list.
   449  // t must not have type arguments.
   450  func (t *Named) AddMethod(m *Func) {
   451  	assert(t.inst == nil)
   452  	t.resolve()
   453  	if i, _ := lookupMethod(t.methods, m.pkg, m.name, false); i < 0 {
   454  		t.methods = append(t.methods, m)
   455  	}
   456  }
   457  
   458  // TODO(gri) Investigate if Unalias can be moved to where underlying is set.
   459  func (t *Named) Underlying() Type { return Unalias(t.resolve().underlying) }
   460  func (t *Named) String() string   { return TypeString(t, nil) }
   461  
   462  // ----------------------------------------------------------------------------
   463  // Implementation
   464  //
   465  // TODO(rfindley): reorganize the loading and expansion methods under this
   466  // heading.
   467  
   468  // under returns the expanded underlying type of n0; possibly by following
   469  // forward chains of named types. If an underlying type is found, resolve
   470  // the chain by setting the underlying type for each defined type in the
   471  // chain before returning it. If no underlying type is found or a cycle
   472  // is detected, the result is Typ[Invalid]. If a cycle is detected and
   473  // n0.check != nil, the cycle is reported.
   474  //
   475  // This is necessary because the underlying type of named may be itself a
   476  // named type that is incomplete:
   477  //
   478  //	type (
   479  //		A B
   480  //		B *C
   481  //		C A
   482  //	)
   483  //
   484  // The type of C is the (named) type of A which is incomplete,
   485  // and which has as its underlying type the named type B.
   486  func (n0 *Named) under() Type {
   487  	u := n0.Underlying()
   488  
   489  	// If the underlying type of a defined type is not a defined
   490  	// (incl. instance) type, then that is the desired underlying
   491  	// type.
   492  	var n1 *Named
   493  	switch u1 := u.(type) {
   494  	case nil:
   495  		// After expansion via Underlying(), we should never encounter a nil
   496  		// underlying.
   497  		panic("nil underlying")
   498  	default:
   499  		// common case
   500  		return u
   501  	case *Named:
   502  		// handled below
   503  		n1 = u1
   504  	}
   505  
   506  	if n0.check == nil {
   507  		panic("Named.check == nil but type is incomplete")
   508  	}
   509  
   510  	// Invariant: after this point n0 as well as any named types in its
   511  	// underlying chain should be set up when this function exits.
   512  	check := n0.check
   513  	n := n0
   514  
   515  	seen := make(map[*Named]int) // types that need their underlying type resolved
   516  	var path []Object            // objects encountered, for cycle reporting
   517  
   518  loop:
   519  	for {
   520  		seen[n] = len(seen)
   521  		path = append(path, n.obj)
   522  		n = n1
   523  		if i, ok := seen[n]; ok {
   524  			// cycle
   525  			check.cycleError(path[i:])
   526  			u = Typ[Invalid]
   527  			break
   528  		}
   529  		u = n.Underlying()
   530  		switch u1 := u.(type) {
   531  		case nil:
   532  			u = Typ[Invalid]
   533  			break loop
   534  		default:
   535  			break loop
   536  		case *Named:
   537  			// Continue collecting *Named types in the chain.
   538  			n1 = u1
   539  		}
   540  	}
   541  
   542  	for n := range seen {
   543  		// We should never have to update the underlying type of an imported type;
   544  		// those underlying types should have been resolved during the import.
   545  		// Also, doing so would lead to a race condition (was go.dev/issue/31749).
   546  		// Do this check always, not just in debug mode (it's cheap).
   547  		if n.obj.pkg != check.pkg {
   548  			panic("imported type with unresolved underlying type")
   549  		}
   550  		n.underlying = u
   551  	}
   552  
   553  	return u
   554  }
   555  
   556  func (n *Named) lookupMethod(pkg *Package, name string, foldCase bool) (int, *Func) {
   557  	n.resolve()
   558  	// If n is an instance, we may not have yet instantiated all of its methods.
   559  	// Look up the method index in orig, and only instantiate method at the
   560  	// matching index (if any).
   561  	i, _ := lookupMethod(n.Origin().methods, pkg, name, foldCase)
   562  	if i < 0 {
   563  		return -1, nil
   564  	}
   565  	// For instances, m.Method(i) will be different from the orig method.
   566  	return i, n.Method(i)
   567  }
   568  
   569  // context returns the type-checker context.
   570  func (check *Checker) context() *Context {
   571  	if check.ctxt == nil {
   572  		check.ctxt = NewContext()
   573  	}
   574  	return check.ctxt
   575  }
   576  
   577  // expandUnderlying substitutes type arguments in the underlying type n.orig,
   578  // returning the result. Returns Typ[Invalid] if there was an error.
   579  func (n *Named) expandUnderlying() Type {
   580  	check := n.check
   581  	if check != nil && check.conf._Trace {
   582  		check.trace(n.obj.pos, "-- Named.expandUnderlying %s", n)
   583  		check.indent++
   584  		defer func() {
   585  			check.indent--
   586  			check.trace(n.obj.pos, "=> %s (tparams = %s, under = %s)", n, n.tparams.list(), n.underlying)
   587  		}()
   588  	}
   589  
   590  	assert(n.inst.orig.underlying != nil)
   591  	if n.inst.ctxt == nil {
   592  		n.inst.ctxt = NewContext()
   593  	}
   594  
   595  	orig := n.inst.orig
   596  	targs := n.inst.targs
   597  
   598  	if asNamed(orig.underlying) != nil {
   599  		// We should only get a Named underlying type here during type checking
   600  		// (for example, in recursive type declarations).
   601  		assert(check != nil)
   602  	}
   603  
   604  	if orig.tparams.Len() != targs.Len() {
   605  		// Mismatching arg and tparam length may be checked elsewhere.
   606  		return Typ[Invalid]
   607  	}
   608  
   609  	// Ensure that an instance is recorded before substituting, so that we
   610  	// resolve n for any recursive references.
   611  	h := n.inst.ctxt.instanceHash(orig, targs.list())
   612  	n2 := n.inst.ctxt.update(h, orig, n.TypeArgs().list(), n)
   613  	assert(n == n2)
   614  
   615  	smap := makeSubstMap(orig.tparams.list(), targs.list())
   616  	var ctxt *Context
   617  	if check != nil {
   618  		ctxt = check.context()
   619  	}
   620  	underlying := n.check.subst(n.obj.pos, orig.underlying, smap, n, ctxt)
   621  	// If the underlying type of n is an interface, we need to set the receiver of
   622  	// its methods accurately -- we set the receiver of interface methods on
   623  	// the RHS of a type declaration to the defined type.
   624  	if iface, _ := underlying.(*Interface); iface != nil {
   625  		if methods, copied := replaceRecvType(iface.methods, orig, n); copied {
   626  			// If the underlying type doesn't actually use type parameters, it's
   627  			// possible that it wasn't substituted. In this case we need to create
   628  			// a new *Interface before modifying receivers.
   629  			if iface == orig.underlying {
   630  				old := iface
   631  				iface = check.newInterface()
   632  				iface.embeddeds = old.embeddeds
   633  				assert(old.complete) // otherwise we are copying incomplete data
   634  				iface.complete = old.complete
   635  				iface.implicit = old.implicit // should be false but be conservative
   636  				underlying = iface
   637  			}
   638  			iface.methods = methods
   639  			iface.tset = nil // recompute type set with new methods
   640  
   641  			// If check != nil, check.newInterface will have saved the interface for later completion.
   642  			if check == nil { // golang/go#61561: all newly created interfaces must be fully evaluated
   643  				iface.typeSet()
   644  			}
   645  		}
   646  	}
   647  
   648  	return underlying
   649  }
   650  
   651  // safeUnderlying returns the underlying type of typ without expanding
   652  // instances, to avoid infinite recursion.
   653  //
   654  // TODO(rfindley): eliminate this function or give it a better name.
   655  func safeUnderlying(typ Type) Type {
   656  	if t := asNamed(typ); t != nil {
   657  		return t.underlying
   658  	}
   659  	return typ.Underlying()
   660  }
   661  

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