go/types: break cycles in invalid types

This is a clean port of CL 354329 from types2 to go/types.

For #48819.

Change-Id: I9efdcdbfa6432f3cee64d924a4c67ecc6793cf86
Reviewed-on: https://go-review.googlesource.com/c/go/+/354349
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/types/decl.go

@@ -356,6 +356,7 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 				if tn == t.obj {
 					check.cycleError(path[i:])
 					t.info = invalid
+					t.underlying = Typ[Invalid]
 					return t.info
 				}
 			}

src/go/types/testdata/fixedbugs/issue41124.go2

@@ -12,7 +12,7 @@ type Nat /* ERROR cycle */ interface {
 
 type Zero struct{}
 type Succ struct{
-	Nat /* ERROR interface contains type constraints */
+	Nat // Nat contains type constraints but is invalid, so no error
 }
 
 // Struct tests.

src/go/types/testdata/fixedbugs/issue48819.src

@@ -0,0 +1,15 @@
+// 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
+
+import "unsafe"
+
+type T /* ERROR illegal cycle in declaration of T */ struct {
+	T
+}
+
+func _(t T) {
+	_ = unsafe.Sizeof(t) // should not go into infinite recursion here
+}

src/go/types/typeset.go

@@ -292,7 +292,7 @@ func computeInterfaceTypeSet(check *Checker, pos token.Pos, ityp *Interface) *_T
 			// Union parsing reports a (delayed) error, so we can ignore this entry.
 			continue
 		default:
-			if typ == Typ[Invalid] {
+			if u == Typ[Invalid] {
 				continue
 			}
 			if check != nil && !check.allowVersion(check.pkg, 1, 18) {