[release-branch.go1.17] go/types: break cycles in invalid types

This is a partial port of CL 354329 from types2 to go/types.
It contains an adjustment to type.go to deal with possibly
invalid type bounds.

Fixes #48825.
For #48819.

Change-Id: I9efdcdbfa6432f3cee64d924a4c67ecc6793cf86
Reviewed-on: https://go-review.googlesource.com/c/go/+/354349
Trust: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/368456
Run-TryBot: Robert Griesemer <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: griesemer

src/go/types/decl.go

@@ -343,6 +343,7 @@ 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/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/type.go

@@ -759,6 +759,9 @@ func (check *Checker) newTypeParam(obj *TypeName, index int, bound Type) *_TypeP
 
 func (t *_TypeParam) Bound() *Interface {
 	iface := asInterface(t.bound)
+	if iface == nil {
+		return &emptyInterface
+	}
 	// use the type bound position if we have one
 	pos := token.NoPos
 	if n, _ := t.bound.(*Named); n != nil {