cmd/compile: in typ0(), load base type before checking s.Def

The loading of the base type in typ0() may cause s.Def to be defined for
the instantiated type, so load the base type before checking s.Def.

Fixes #50486

Change-Id: Ic039bc8f774dda534f4ccd1f920220b7a10dede6
Reviewed-on: https://go-review.googlesource.com/c/go/+/377094
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/noder/types.go

@@ -113,6 +113,15 @@ func (g *irgen) typ0(typ types2.Type) *types.Type {
 			// based on the names of the type arguments.
 			instName := g.instTypeName2(typ.Obj().Name(), typ.TypeArgs())
 			s := g.pkg(typ.Obj().Pkg()).Lookup(instName)
+
+			// Make sure the base generic type exists in type1 (it may
+			// not yet if we are referecing an imported generic type, as
+			// opposed to a generic type declared in this package). Make
+			// sure to do this lookup before checking s.Def, in case
+			// s.Def gets defined while importing base (if an imported
+			// type). (Issue #50486).
+			base := g.obj(typ.Origin().Obj())
+
 			if s.Def != nil {
 				// We have already encountered this instantiation.
 				// Use the type we previously created, since there
@@ -120,10 +129,6 @@ func (g *irgen) typ0(typ types2.Type) *types.Type {
 				return s.Def.Type()
 			}
 
-			// Make sure the base generic type exists in type1 (it may
-			// not yet if we are referecing an imported generic type, as
-			// opposed to a generic type declared in this package).
-			base := g.obj(typ.Origin().Obj())
 			if base.Class == ir.PAUTO {
 				// If the base type is a local type, we want to pop
 				// this instantiated type symbol/definition when we

test/typeparam/issue50486.dir/goerror_fp.go

@@ -0,0 +1,75 @@
+// 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 goerror_fp
+
+type Seq[T any] []T
+
+func (r Seq[T]) Size() int {
+	return len(r)
+}
+
+func (r Seq[T]) Append(items ...T) Seq[T] {
+	tail := Seq[T](items)
+	ret := make(Seq[T], r.Size()+tail.Size())
+
+	for i := range r {
+		ret[i] = r[i]
+	}
+
+	for i := range tail {
+		ret[i+r.Size()] = tail[i]
+	}
+
+	return ret
+}
+
+func (r Seq[T]) Iterator() Iterator[T] {
+	idx := 0
+
+	return Iterator[T]{
+		IsHasNext: func() bool {
+			return idx < r.Size()
+		},
+		GetNext: func() T {
+			ret := r[idx]
+			idx++
+			return ret
+		},
+	}
+}
+
+type Iterator[T any] struct {
+	IsHasNext func() bool
+	GetNext   func() T
+}
+
+func (r Iterator[T]) ToSeq() Seq[T] {
+	ret := Seq[T]{}
+	for r.HasNext() {
+		ret = append(ret, r.Next())
+	}
+	return ret
+}
+
+func (r Iterator[T]) Map(f func(T) any) Iterator[any] {
+	return MakeIterator(r.HasNext, func() any {
+		return f(r.Next())
+	})
+}
+
+func (r Iterator[T]) HasNext() bool {
+	return r.IsHasNext()
+}
+
+func (r Iterator[T]) Next() T {
+	return r.GetNext()
+}
+
+func MakeIterator[T any](has func() bool, next func() T) Iterator[T] {
+	return Iterator[T]{
+		IsHasNext: has,
+		GetNext:   next,
+	}
+}

test/typeparam/issue50486.dir/main.go

@@ -0,0 +1,16 @@
+package main
+
+import fp "goerror_fp"
+
+func Fold[A, B any](zero B, a A, f func(B, A) B) B {
+	return f(zero, a)
+}
+
+func main() {
+
+	var v any = "hello"
+	Fold(fp.Seq[any]{}, v, func(seq fp.Seq[any], v any) fp.Seq[any] {
+		return seq.Append(v)
+	})
+
+}

test/typeparam/issue50486.go

@@ -0,0 +1,7 @@
+// compiledir -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