go/types,types2: disallow illegal cycles through Unions

Checker.validType was not considering Unions when looking for illegal
cycles.

Fixes #48582

Change-Id: I11ad0279eeaaa56bb6d5731b0572c1c3a0c459eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/351829
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

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

@@ -315,6 +315,13 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 			}
 		}
 
+	case *Union:
+		for _, t := range t.terms {
+			if check.validType(t.typ, path) == invalid {
+				return invalid
+			}
+		}
+
 	case *Interface:
 		for _, etyp := range t.embeddeds {
 			if check.validType(etyp, path) == invalid {

src/cmd/compile/internal/types2/testdata/fixedbugs/issue41124.go2

@@ -6,7 +6,7 @@ package p
 
 // Test case from issue.
 
-type Nat interface {
+type Nat /* ERROR cycle */ interface {
 	Zero|Succ
 }
 

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48582.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
+
+type N /* ERROR cycle */ interface {
+	int | N
+}
+
+type A /* ERROR cycle */ interface {
+	int | B
+}
+
+type B interface {
+	int | A
+}
+
+type S /* ERROR cycle */ struct {
+	I // ERROR interface contains type constraints
+}
+
+type I interface {
+	int | S
+}
+
+type P interface {
+	*P // ERROR interface contains type constraints
+}

src/go/types/decl.go

@@ -314,6 +314,13 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 			}
 		}
 
+	case *Union:
+		for _, t := range t.terms {
+			if check.validType(t.typ, path) == invalid {
+				return invalid
+			}
+		}
+
 	case *Interface:
 		for _, etyp := range t.embeddeds {
 			if check.validType(etyp, path) == invalid {

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

@@ -6,13 +6,13 @@ package p
 
 // Test case from issue.
 
-type Nat interface {
+type Nat /* ERROR cycle */ interface {
 	Zero|Succ
 }
 
 type Zero struct{}
 type Succ struct{
-	Nat // ERROR interface contains type constraints
+	Nat /* ERROR interface contains type constraints */
 }
 
 // Struct tests.

src/go/types/testdata/fixedbugs/issue48582.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
+
+type N /* ERROR cycle */ interface {
+	int | N
+}
+
+type A /* ERROR cycle */ interface {
+	int | B
+}
+
+type B interface {
+	int | A
+}
+
+type S /* ERROR cycle */ struct {
+	I // ERROR interface contains type constraints
+}
+
+type I interface {
+	int | S
+}
+
+type P interface {
+	*P // ERROR interface contains type constraints
+}