cmd/compile: mark type descriptors as always dupok

The types of the two interfaces should be equal, but they aren't.
We end up with multiple descriptors for a type when we need type
descriptors to be unique.

Fixes #49241

Change-Id: I8a6c70da541c6088a92a01392bc83b61cc130eba
Reviewed-on: https://go-review.googlesource.com/c/go/+/360134
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -959,11 +959,6 @@ func writeType(t *types.Type) *obj.LSym {
 		base.Fatalf("unresolved defined type: %v", tbase)
 	}
 
-	dupok := 0
-	if tbase.Sym() == nil || tbase.HasShape() { // TODO(mdempsky): Probably need DUPOK for instantiated types too.
-		dupok = obj.DUPOK
-	}
-
 	if !NeedEmit(tbase) {
 		if i := typecheck.BaseTypeIndex(t); i >= 0 {
 			lsym.Pkg = tbase.Sym().Pkg.Prefix
@@ -1196,7 +1191,9 @@ func writeType(t *types.Type) *obj.LSym {
 	}
 
 	ot = dextratypeData(lsym, ot, t)
-	objw.Global(lsym, int32(ot), int16(dupok|obj.RODATA))
+	objw.Global(lsym, int32(ot), int16(obj.DUPOK|obj.RODATA))
+	// Note: DUPOK is required to ensure that we don't end up with more
+	// than one type descriptor for a given type.
 
 	// The linker will leave a table of all the typelinks for
 	// types in the binary, so the runtime can find them.

test/typeparam/issue49241.dir/a.go

@@ -0,0 +1,13 @@
+// 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 T[P any] struct {
+	x P
+}
+
+type U struct {
+	a,b int
+}

test/typeparam/issue49241.dir/b.go

@@ -0,0 +1,17 @@
+// 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 b
+
+import "a"
+
+//go:noinline
+func F() interface {} {
+	return a.T[int]{}
+}
+
+//go:noinline
+func G() interface{} {
+	return struct{X,Y a.U}{}
+}

test/typeparam/issue49241.dir/c.go

@@ -0,0 +1,17 @@
+// 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 c
+
+import "a"
+
+//go:noinline
+func F() interface {} {
+	return a.T[int]{}
+}
+
+//go:noinline
+func G() interface{} {
+	return struct{X,Y a.U}{}
+}

test/typeparam/issue49241.dir/main.go

@@ -0,0 +1,21 @@
+// 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 (
+	"b"
+	"c"
+)
+
+func main() {
+	if b.G() != c.G() {
+		println(b.G(), c.G())
+		panic("bad")
+	}
+	if b.F() != c.F() {
+		println(b.F(), c.F())
+		panic("bad")
+	}
+}

test/typeparam/issue49241.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