1 // Copyright 2022 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.
7 // validType verifies that the given type does not "expand" indefinitely
8 // producing a cycle in the type graph.
9 // (Cycles involving alias types, as in "type A = [10]A" are detected
10 // earlier, via the objDecl cycle detection mechanism.)
11 func (check *Checker) validType(typ *Named) {
12 check.validType0(typ, nil, nil, nil)
15 // validType0 checks if the given type is valid. If typ is a type parameter
16 // its value is looked up in the provided environment. The environment is
17 // nil if typ is not part of (the RHS of) an instantiated type, in that case
18 // any type parameter encountered must be from an enclosing function and can
19 // be ignored. The nest list describes the stack (the "nest in memory") of
20 // types which contain (or embed in the case of interfaces) other types. For
21 // instance, a struct named S which contains a field of named type F contains
22 // (the memory of) F in S, leading to the nest S->F. If a type appears in its
23 // own nest (say S->F->S) we have an invalid recursive type. The path list is
24 // the full path of named types in a cycle, it is only needed for error reporting.
25 func (check *Checker) validType0(typ Type, env *tparamEnv, nest, path []*Named) bool {
26 switch t := typ.(type) {
28 // We should never see a nil type but be conservative and panic
29 // only in debug mode.
31 panic("validType0(nil)")
35 return check.validType0(t.elem, env, nest, path)
38 for _, f := range t.fields {
39 if !check.validType0(f.typ, env, nest, path) {
45 for _, t := range t.terms {
46 if !check.validType0(t.typ, env, nest, path) {
52 for _, etyp := range t.embeddeds {
53 if !check.validType0(etyp, env, nest, path) {
59 // Exit early if we already know t is valid.
60 // This is purely an optimization but it prevents excessive computation
61 // times in pathological cases such as testdata/fixedbugs/issue6977.go.
62 // (Note: The valids map could also be allocated locally, once for each
64 if check.valids.lookup(t) != nil {
68 // Don't report a 2nd error if we already know the type is invalid
69 // (e.g., if a cycle was detected earlier, via under).
70 // Note: ensure that t.orig is fully resolved by calling Underlying().
71 if t.Underlying() == Typ[Invalid] {
75 // If the current type t is also found in nest, (the memory of) t is
76 // embedded in itself, indicating an invalid recursive type.
77 for _, e := range nest {
79 // t cannot be in an imported package otherwise that package
80 // would have reported a type cycle and couldn't have been
81 // imported in the first place.
82 assert(t.obj.pkg == check.pkg)
83 t.underlying = Typ[Invalid] // t is in the current package (no race possibility)
84 // Find the starting point of the cycle and report it.
85 // Because each type in nest must also appear in path (see invariant below),
86 // type t must be in path since it was found in nest. But not every type in path
87 // is in nest. Specifically t may appear in path with an earlier index than the
88 // index of t in nest. Search again.
89 for start, p := range path {
91 check.cycleError(makeObjList(path[start:]))
95 panic("cycle start not found")
99 // No cycle was found. Check the RHS of t.
100 // Every type added to nest is also added to path; thus every type that is in nest
101 // must also be in path (invariant). But not every type in path is in nest, since
102 // nest may be pruned (see below, *TypeParam case).
103 if !check.validType0(t.Origin().fromRHS, env.push(t), append(nest, t), append(path, t)) {
107 check.valids.add(t) // t is valid
110 // A type parameter stands for the type (argument) it was instantiated with.
111 // Check the corresponding type argument for validity if we have one.
113 if targ := env.tmap[t]; targ != nil {
114 // Type arguments found in targ must be looked
115 // up in the enclosing environment env.link. The
116 // type argument must be valid in the enclosing
117 // type (where the current type was instantiated),
118 // hence we must check targ's validity in the type
119 // nest excluding the current (instantiated) type
120 // (see the example at the end of this file).
121 // For error reporting we keep the full path.
122 return check.validType0(targ, env.link, nest[:len(nest)-1], path)
130 // makeObjList returns the list of type name objects for the given
131 // list of named types.
132 func makeObjList(tlist []*Named) []Object {
133 olist := make([]Object, len(tlist))
134 for i, t := range tlist {
140 // A tparamEnv provides the environment for looking up the type arguments
141 // with which type parameters for a given instance were instantiated.
142 // If we don't have an instance, the corresponding tparamEnv is nil.
143 type tparamEnv struct {
148 func (env *tparamEnv) push(typ *Named) *tparamEnv {
149 // If typ is not an instantiated type there are no typ-specific
150 // type parameters to look up and we don't need an environment.
151 targs := typ.TypeArgs()
153 return nil // no instance => nil environment
156 // Populate tmap: remember the type argument for each type parameter.
157 // We cannot use makeSubstMap because the number of type parameters
158 // and arguments may not match due to errors in the source (too many
159 // or too few type arguments). Populate tmap "manually".
160 tparams := typ.TypeParams()
161 n, m := targs.Len(), tparams.Len()
163 n = m // too many targs
165 tmap := make(substMap, n)
166 for i := 0; i < n; i++ {
167 tmap[tparams.At(i)] = targs.At(i)
170 return &tparamEnv{tmap: tmap, link: env}
173 // TODO(gri) Alternative implementation:
174 // We may not need to build a stack of environments to
175 // look up the type arguments for type parameters. The
176 // same information should be available via the path:
177 // We should be able to just walk the path backwards
178 // and find the type arguments in the instance objects.
180 // Here is an example illustrating why we need to exclude the
181 // instantiated type from nest when evaluating the validity of
182 // a type parameter. Given the declarations
184 // var _ A[A[string]]
186 // type A[P any] struct { _ B[P] }
187 // type B[P any] struct { _ P }
189 // we want to determine if the type A[A[string]] is valid.
190 // We start evaluating A[A[string]] outside any type nest:
196 // The RHS of A is now evaluated in the A[A[string]] nest:
199 // nest = A[A[string]]
200 // path = A[A[string]]
202 // The struct has a single field of type B[P₁] with which
206 // nest = A[A[string]]
207 // path = A[A[string]]
210 // nest = A[A[string]]->B[P]
211 // path = A[A[string]]->B[P]
213 // Eventutally we reach the type parameter P of type B (P₂):
216 // nest = A[A[string]]->B[P]
217 // path = A[A[string]]->B[P]
219 // The type argument for P of B is the type parameter P of A (P₁).
220 // It must be evaluated in the type nest that existed when B was
224 // nest = A[A[string]] <== type nest at B's instantiation time
225 // path = A[A[string]]->B[P]
227 // If we'd use the current nest it would correspond to the path
228 // which will be wrong as we will see shortly. P's type argument
229 // is A[string], which again must be evaluated in the type nest
230 // that existed when A was instantiated with A[string]. That type
234 // nest = <== type nest at A's instantiation time
235 // path = A[A[string]]->B[P]
237 // Evaluation then proceeds as before for A[string]:
241 // path = A[A[string]]->B[P]->A[string]
243 // Now we reach B[P] again. If we had not adjusted nest, it would
244 // correspond to path, and we would find B[P] in nest, indicating
245 // a cycle, which would clearly be wrong since there's no cycle in
250 // path = A[A[string]]->B[P]->A[string] <== path contains B[P]!
252 // But because we use the correct type nest, evaluation proceeds without
253 // errors and we get the evaluation sequence:
256 // nest = A[string]->B[P]
257 // path = A[A[string]]->B[P]->A[string]->B[P]
259 // nest = A[string]->B[P]
260 // path = A[A[string]]->B[P]->A[string]->B[P]
263 // path = A[A[string]]->B[P]->A[string]->B[P]
266 // path = A[A[string]]->B[P]->A[string]->B[P]
268 // At this point we're done and A[A[string]] and is valid.