[dev.typeparams] cmd/compile/internal/types2: fix generic type indirection

Change-Id: If25ceb2aa403b94608760be331faa2aff11c47cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/333890
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/expr.go

@@ -1479,13 +1479,24 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 				case typexpr:
 					x.typ = &Pointer{base: x.typ}
 				default:
-					if typ := asPointer(x.typ); typ != nil {
-						x.mode = variable
-						x.typ = typ.base
-					} else {
-						check.errorf(x, invalidOp+"cannot indirect %s", x)
+					var base Type
+					if !underIs(x.typ, func(u Type) bool {
+						p, _ := u.(*Pointer)
+						if p == nil {
+							check.errorf(x, invalidOp+"cannot indirect %s", x)
+							return false
+						}
+						if base != nil && !Identical(p.base, base) {
+							check.errorf(x, invalidOp+"pointers of %s must have identical base types", x)
+							return false
+						}
+						base = p.base
+						return true
+					}) {
 						goto Error
 					}
+					x.mode = variable
+					x.typ = base
 				}
 				break
 			}

src/cmd/compile/internal/types2/testdata/examples/operations.go2

@@ -0,0 +1,29 @@
+// 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 p
+
+// indirection
+
+func _[P any](p P) {
+        _ = *p // ERROR cannot indirect p
+}
+
+func _[P interface{ int }](p P) {
+        _ = *p // ERROR cannot indirect p
+}
+
+func _[P interface{ *int }](p P) {
+        _ = *p
+}
+
+func _[P interface{ *int | *string }](p P) {
+        _ = *p // ERROR must have identical base types
+}
+
+type intPtr *int
+
+func _[P interface{ *int | intPtr } ](p P) {
+        var _ int = *p
+}