[dev.typeparams] go/types: fix generic type indirection

This is a port of CL 333890 to go/types.

Change-Id: I8ee20f405dad98083bb5e91636044d132a95d909
Reviewed-on: https://go-review.googlesource.com/c/go/+/335081
Trust: Robert Findley <[email protected]>
Run-TryBot: Robert Findley <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>

Author: findleyr

src/go/types/expr.go

@@ -1400,13 +1400,24 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		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.invalidOp(x, _InvalidIndirection, "cannot indirect %s", x)
+			var base Type
+			if !underIs(x.typ, func(u Type) bool {
+				p, _ := u.(*Pointer)
+				if p == nil {
+					check.invalidOp(x, _InvalidIndirection, "cannot indirect %s", x)
+					return false
+				}
+				if base != nil && !Identical(p.base, base) {
+					check.invalidOp(x, _Todo, "pointers of %s must have identical base types", x)
+					return false
+				}
+				base = p.base
+				return true
+			}) {
 				goto Error
 			}
+			x.mode = variable
+			x.typ = base
 		}
 
 	case *ast.UnaryExpr:

src/go/types/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
+}