cmd/compile: some fixes in type substituter for Instantiate

In the case in (*TSubster).Type() that we were running into an
incomplete underlying type (TFORW), we should just be immediately
returning the type returned by ts.SubstForwFunc(forw), since that call
returns a proper type node, and has set up any remaining work that has
to be done when we get done with the current top-level type definition.
(For import, that function is doInst, which does an Instantiate of the
new substituted type, with the delayed part via deferredInstStack.) We
should not continue doing the later parts of (*TSubster).Type(), since
the underlying type may not yet have its methods filled in, etc.

Also, in Instantiate(), we need to put the desired new type on
deferredInstStack, even if the base type node already exists, if the
type node is in TFORW state. This is now exactly the case when
Instantiate is called from (*TSubster).Type via doInst, since
(*TSubster).Type has already called NewIncompleteNamedType().

Fixes #48716
Fixes #48889

Change-Id: Icd6be5721c4ac75bf8869b8bbdeca50069d632ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/355250
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/typecheck/iimport.go

@@ -1822,9 +1822,25 @@ func Instantiate(pos src.XPos, baseType *types.Type, targs []*types.Type) *types
 	instSym := baseSym.Pkg.Lookup(name)
 	if instSym.Def != nil {
 		// May match existing type from previous import or
-		// types2-to-types1 conversion, or from in-progress instantiation
-		// in the current type import stack.
-		return instSym.Def.Type()
+		// types2-to-types1 conversion.
+		t := instSym.Def.Type()
+		if t.Kind() != types.TFORW {
+			return t
+		}
+		// Or, we have started creating this type in (*TSubster).Typ, but its
+		// underlying type was not completed yet, so we need to add this type
+		// to deferredInstStack, if not already there.
+		found := false
+		for _, t2 := range deferredInstStack {
+			if t2 == t {
+				found = true
+				break
+			}
+		}
+		if !found {
+			deferredInstStack = append(deferredInstStack, t)
+		}
+		return t
 	}
 
 	t := NewIncompleteNamedType(baseType.Pos(), instSym)
@@ -1865,6 +1881,7 @@ func resumeDoInst() {
 // during a type substitution for an instantiation. This is needed for
 // instantiations of mutually recursive types.
 func doInst(t *types.Type) *types.Type {
+	assert(t.Kind() == types.TFORW)
 	return Instantiate(t.Pos(), t.OrigSym().Def.(*ir.Name).Type(), t.RParams())
 }
 
@@ -1873,6 +1890,7 @@ func doInst(t *types.Type) *types.Type {
 // instantiation being created, baseType is the base generic type, and targs are
 // the type arguments that baseType is being instantiated with.
 func substInstType(t *types.Type, baseType *types.Type, targs []*types.Type) {
+	assert(t.Kind() == types.TFORW)
 	subst := Tsubster{
 		Tparams:       baseType.RParams(),
 		Targs:         targs,

src/cmd/compile/internal/typecheck/subr.go

@@ -1217,7 +1217,7 @@ func (ts *Tsubster) typ1(t *types.Type) *types.Type {
 		}
 	case types.TFORW:
 		if ts.SubstForwFunc != nil {
-			newt = ts.SubstForwFunc(t)
+			return ts.SubstForwFunc(forw)
 		} else {
 			assert(false)
 		}

test/typeparam/issue48716.dir/a.go

@@ -0,0 +1,51 @@
+// Copyright 2021 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 a
+
+type Pair[L, R any] struct {
+	L L
+	R R
+}
+
+func Two[L, R any](l L, r R) Pair[L, R] {
+	return Pair[L, R]{L: l, R: r}
+}
+
+type Map[K, V any] interface {
+	Put(K, V)
+	Len() int
+	Iterate(func(Pair[K, V]) bool)
+}
+
+type HashMap[K comparable, V any] struct {
+	m map[K]V
+}
+
+func NewHashMap[K comparable, V any](capacity int) HashMap[K, V] {
+	var m map[K]V
+	if capacity >= 1 {
+		m = make(map[K]V, capacity)
+	} else {
+		m = map[K]V{}
+	}
+
+	return HashMap[K, V]{m: m}
+}
+
+func (m HashMap[K, V]) Put(k K, v V) {
+	m.m[k] = v
+}
+
+func (m HashMap[K, V]) Len() int {
+	return len(m.m)
+}
+
+func (m HashMap[K, V]) Iterate(cb func(Pair[K, V]) bool) {
+	for k, v := range m.m {
+		if !cb(Two(k, v)) {
+			return
+		}
+	}
+}

test/typeparam/issue48716.dir/main.go

@@ -0,0 +1,58 @@
+// Copyright 2021 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 main
+
+import (
+	"a"
+)
+
+// Creates copy of set
+func Copy[T comparable](src MapSet[T]) (dst MapSet[T]) {
+	dst = HashSet[T](src.Len())
+	Fill(src, dst)
+	return
+}
+
+// Fill src from dst
+func Fill[T any](src, dst MapSet[T]) {
+	src.Iterate(func(t T) bool {
+		dst.Add(t)
+		return true
+	})
+	return
+}
+
+type MapSet[T any] struct {
+	m a.Map[T, struct{}]
+}
+
+func HashSet[T comparable](capacity int) MapSet[T] {
+	return FromMap[T](a.NewHashMap[T, struct{}](capacity))
+}
+
+func FromMap[T any](m a.Map[T, struct{}]) MapSet[T] {
+	return MapSet[T]{
+		m: m,
+	}
+}
+
+func (s MapSet[T]) Add(t T) {
+	s.m.Put(t, struct{}{})
+}
+
+func (s MapSet[T]) Len() int {
+	return s.m.Len()
+}
+
+func (s MapSet[T]) Iterate(cb func(T) bool) {
+	s.m.Iterate(func(p a.Pair[T, struct{}]) bool {
+		return cb(p.L)
+	})
+}
+
+func main() {
+	x := FromMap[int](a.NewHashMap[int, struct{}](1))
+	Copy[int](x)
+}

test/typeparam/issue48716.go

@@ -0,0 +1,7 @@
+// rundir -G=3
+
+// Copyright 2021 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 ignored