go/types, types2: introduce _Alias type node

[gostls13.git] / src / cmd / compile / internal / types2 / validtype.go

// Copyright 2022 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package types2

// validType verifies that the given type does not "expand" indefinitely
// producing a cycle in the type graph.
// (Cycles involving alias types, as in "type A = [10]A" are detected
// earlier, via the objDecl cycle detection mechanism.)
func (check *Checker) validType(typ *Named) {
        check.validType0(typ, nil, nil)
}

// validType0 checks if the given type is valid. If typ is a type parameter
// its value is looked up in the type argument list of the instantiated
// (enclosing) type, if it exists. Otherwise the type parameter must be from
// an enclosing function and can be ignored.
// The nest list describes the stack (the "nest in memory") of types which
// contain (or embed in the case of interfaces) other types. For instance, a
// struct named S which contains a field of named type F contains (the memory
// of) F in S, leading to the nest S->F. If a type appears in its own nest
// (say S->F->S) we have an invalid recursive type. The path list is the full
// path of named types in a cycle, it is only needed for error reporting.
func (check *Checker) validType0(typ Type, nest, path []*Named) bool {
        switch t := _Unalias(typ).(type) {
        case nil:
                // We should never see a nil type but be conservative and panic
                // only in debug mode.
                if debug {
                        panic("validType0(nil)")
                }

        case *Array:
                return check.validType0(t.elem, nest, path)

        case *Struct:
                for _, f := range t.fields {
                        if !check.validType0(f.typ, nest, path) {
                                return false
                        }
                }

        case *Union:
                for _, t := range t.terms {
                        if !check.validType0(t.typ, nest, path) {
                                return false
                        }
                }

        case *Interface:
                for _, etyp := range t.embeddeds {
                        if !check.validType0(etyp, nest, path) {
                                return false
                        }
                }

        case *Named:
                // Exit early if we already know t is valid.
                // This is purely an optimization but it prevents excessive computation
                // times in pathological cases such as testdata/fixedbugs/issue6977.go.
                // (Note: The valids map could also be allocated locally, once for each
                // validType call.)
                if check.valids.lookup(t) != nil {
                        break
                }

                // Don't report a 2nd error if we already know the type is invalid
                // (e.g., if a cycle was detected earlier, via under).
                // Note: ensure that t.orig is fully resolved by calling Underlying().
                if !isValid(t.Underlying()) {
                        return false
                }

                // If the current type t is also found in nest, (the memory of) t is
                // embedded in itself, indicating an invalid recursive type.
                for _, e := range nest {
                        if Identical(e, t) {
                                // We have a cycle. If t != t.Origin() then t is an instance of
                                // the generic type t.Origin(). Because t is in the nest, t must
                                // occur within the definition (RHS) of the generic type t.Origin(),
                                // directly or indirectly, after expansion of the RHS.
                                // Therefore t.Origin() must be invalid, no matter how it is
                                // instantiated since the instantiation t of t.Origin() happens
                                // inside t.Origin()'s RHS and thus is always the same and always
                                // present.
                                // Therefore we can mark the underlying of both t and t.Origin()
                                // as invalid. If t is not an instance of a generic type, t and
                                // t.Origin() are the same.
                                // Furthermore, because we check all types in a package for validity
                                // before type checking is complete, any exported type that is invalid
                                // will have an invalid underlying type and we can't reach here with
                                // such a type (invalid types are excluded above).
                                // Thus, if we reach here with a type t, both t and t.Origin() (if
                                // different in the first place) must be from the current package;
                                // they cannot have been imported.
                                // Therefore it is safe to change their underlying types; there is
                                // no chance for a race condition (the types of the current package
                                // are not yet available to other goroutines).
                                assert(t.obj.pkg == check.pkg)
                                assert(t.Origin().obj.pkg == check.pkg)
                                t.underlying = Typ[Invalid]
                                t.Origin().underlying = Typ[Invalid]

                                // Find the starting point of the cycle and report it.
                                // Because each type in nest must also appear in path (see invariant below),
                                // type t must be in path since it was found in nest. But not every type in path
                                // is in nest. Specifically t may appear in path with an earlier index than the
                                // index of t in nest. Search again.
                                for start, p := range path {
                                        if Identical(p, t) {
                                                check.cycleError(makeObjList(path[start:]))
                                                return false
                                        }
                                }
                                panic("cycle start not found")
                        }
                }

                // No cycle was found. Check the RHS of t.
                // Every type added to nest is also added to path; thus every type that is in nest
                // must also be in path (invariant). But not every type in path is in nest, since
                // nest may be pruned (see below, *TypeParam case).
                if !check.validType0(t.Origin().fromRHS, append(nest, t), append(path, t)) {
                        return false
                }

                check.valids.add(t) // t is valid

        case *TypeParam:
                // A type parameter stands for the type (argument) it was instantiated with.
                // Check the corresponding type argument for validity if we are in an
                // instantiated type.
                if len(nest) > 0 {
                        inst := nest[len(nest)-1] // the type instance
                        // Find the corresponding type argument for the type parameter
                        // and proceed with checking that type argument.
                        for i, tparam := range inst.TypeParams().list() {
                                // The type parameter and type argument lists should
                                // match in length but be careful in case of errors.
                                if t == tparam && i < inst.TypeArgs().Len() {
                                        targ := inst.TypeArgs().At(i)
                                        // The type argument must be valid in the enclosing
                                        // type (where inst was instantiated), hence we must
                                        // check targ's validity in the type nest excluding
                                        // the current (instantiated) type (see the example
                                        // at the end of this file).
                                        // For error reporting we keep the full path.
                                        return check.validType0(targ, nest[:len(nest)-1], path)
                                }
                        }
                }
        }

        return true
}

// makeObjList returns the list of type name objects for the given
// list of named types.
func makeObjList(tlist []*Named) []Object {
        olist := make([]Object, len(tlist))
        for i, t := range tlist {
                olist[i] = t.obj
        }
        return olist
}

// Here is an example illustrating why we need to exclude the
// instantiated type from nest when evaluating the validity of
// a type parameter. Given the declarations
//
//   var _ A[A[string]]
//
//   type A[P any] struct { _ B[P] }
//   type B[P any] struct { _ P }
//
// we want to determine if the type A[A[string]] is valid.
// We start evaluating A[A[string]] outside any type nest:
//
//   A[A[string]]
//         nest =
//         path =
//
// The RHS of A is now evaluated in the A[A[string]] nest:
//
//   struct{_ B[P₁]}
//         nest = A[A[string]]
//         path = A[A[string]]
//
// The struct has a single field of type B[P₁] with which
// we continue:
//
//   B[P₁]
//         nest = A[A[string]]
//         path = A[A[string]]
//
//   struct{_ P₂}
//         nest = A[A[string]]->B[P]
//         path = A[A[string]]->B[P]
//
// Eventually we reach the type parameter P of type B (P₂):
//
//   P₂
//         nest = A[A[string]]->B[P]
//         path = A[A[string]]->B[P]
//
// The type argument for P of B is the type parameter P of A (P₁).
// It must be evaluated in the type nest that existed when B was
// instantiated:
//
//   P₁
//         nest = A[A[string]]        <== type nest at B's instantiation time
//         path = A[A[string]]->B[P]
//
// If we'd use the current nest it would correspond to the path
// which will be wrong as we will see shortly. P's type argument
// is A[string], which again must be evaluated in the type nest
// that existed when A was instantiated with A[string]. That type
// nest is empty:
//
//   A[string]
//         nest =                     <== type nest at A's instantiation time
//         path = A[A[string]]->B[P]
//
// Evaluation then proceeds as before for A[string]:
//
//   struct{_ B[P₁]}
//         nest = A[string]
//         path = A[A[string]]->B[P]->A[string]
//
// Now we reach B[P] again. If we had not adjusted nest, it would
// correspond to path, and we would find B[P] in nest, indicating
// a cycle, which would clearly be wrong since there's no cycle in
// A[string]:
//
//   B[P₁]
//         nest = A[string]
//         path = A[A[string]]->B[P]->A[string]  <== path contains B[P]!
//
// But because we use the correct type nest, evaluation proceeds without
// errors and we get the evaluation sequence:
//
//   struct{_ P₂}
//         nest = A[string]->B[P]
//         path = A[A[string]]->B[P]->A[string]->B[P]
//   P₂
//         nest = A[string]->B[P]
//         path = A[A[string]]->B[P]->A[string]->B[P]
//   P₁
//         nest = A[string]
//         path = A[A[string]]->B[P]->A[string]->B[P]
//   string
//         nest =
//         path = A[A[string]]->B[P]->A[string]->B[P]
//
// At this point we're done and A[A[string]] and is valid.