go/types, types2: handle case of no specific target types in conversion

Avoid a panic by handling the case of no specific target type in a type
parameter to type parameter conversions.

Fixes #49864

Change-Id: I117dd80cc9d47c8c1e168f1caf0f281726270c84
Reviewed-on: https://go-review.googlesource.com/c/go/+/367616
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

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

@@ -233,6 +233,9 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 			}
 			x.typ = V.typ
 			return Tp.is(func(T *term) bool {
+				if T == nil {
+					return false // no specific types
+				}
 				if !x.convertibleTo(check, T.typ, cause) {
 					errorf("cannot convert %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
 					return false

src/cmd/compile/internal/types2/testdata/fixedbugs/issue49864.go2

@@ -0,0 +1,9 @@
+// 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
+
+func _[P ~int, Q any](p P) {
+	_ = Q(p /* ERROR cannot convert */ )
+}

src/go/types/conversions.go

@@ -224,6 +224,9 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 			}
 			x.typ = V.typ
 			return Tp.is(func(T *term) bool {
+				if T == nil {
+					return false // no specific types
+				}
 				if !x.convertibleTo(check, T.typ, cause) {
 					errorf("cannot convert %s (in %s) to %s (in %s)", V.typ, Vp, T.typ, Tp)
 					return false

src/go/types/testdata/fixedbugs/issue49864.go2

@@ -0,0 +1,9 @@
+// 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
+
+func _[P ~int, Q any](p P) {
+	_ = Q(p /* ERROR cannot convert */ )
+}