[release-branch.go1.18] cmd/compile: pointers to notinheap types need their own shape

They should not share a shape with regular pointers. We could coalesce
multiple pointer-to-not-in-heap types, but doesn't seem worth it - just
make them fully stenciled.

Fixes #51741

Change-Id: Ie8158177226fbc46a798e71c51897a82f15153df
Reviewed-on: https://go-review.googlesource.com/c/go/+/393895
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
(cherry picked from commit fcf6afb82d)
Reviewed-on: https://go-review.googlesource.com/c/go/+/393935

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

@@ -1536,7 +1536,7 @@ func Shapify(t *types.Type, index int, tparam *types.Type) *types.Type {
 	// Note: pointers to arrays are special because of slice-to-array-pointer
 	// conversions. See issue 49295.
 	if u.Kind() == types.TPTR && u.Elem().Kind() != types.TARRAY &&
-		tparam.Bound().StructuralType() == nil {
+		tparam.Bound().StructuralType() == nil && !u.Elem().NotInHeap() {
 		u = types.Types[types.TUINT8].PtrTo()
 	}
 

test/typeparam/issue51733.go

@@ -0,0 +1,32 @@
+// run -gcflags=-G=3
+
+// 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 main
+
+import (
+	"log"
+	"unsafe"
+)
+
+//go:notinheap
+type S struct{}
+
+func main() {
+	p := (*S)(unsafe.Pointer(uintptr(0x8000)))
+	var v any = p
+	p2 := v.(*S)
+	if p != p2 {
+		log.Fatalf("%p != %p", unsafe.Pointer(p), unsafe.Pointer(p2))
+	}
+	p2 = typeAssert[*S](v)
+	if p != p2 {
+		log.Fatalf("%p != %p from typeAssert", unsafe.Pointer(p), unsafe.Pointer(p2))
+	}
+}
+
+func typeAssert[T any](v any) T {
+	return v.(T)
+}