[dev.go2go] go/types: report "inifinite expansion" cycles across instantiated types

Fixes #39938.

Change-Id: I482b211189cefdc098df5acb4f858f6f1635ce60
Reviewed-on: https://go-review.googlesource.com/c/go/+/240519
Reviewed-by: Robert Griesemer <[email protected]>

Author: griesemer

src/go/types/check_test.go

@@ -133,6 +133,7 @@ var tests = [][]string{
 	{"fixedbugs/issue39754.go2"},
 	{"fixedbugs/issue39755.go2"},
 	{"fixedbugs/issue39768.go2"},
+	{"fixedbugs/issue39938.go2"},
 }
 
 var fset = token.NewFileSet()

src/go/types/decl.go

@@ -349,6 +349,9 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 			panic("internal error: cycle start not found")
 		}
 		return t.info
+
+	case *instance:
+		return check.validType(t.expand(), path)
 	}
 
 	return valid

src/go/types/fixedbugs/issue39938.go2

@@ -0,0 +1,50 @@
+// Copyright 2020 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.
+
+// Check "infinite expansion" cycle errors across instantiated types.
+
+package p
+
+type E0(type P) P
+type E1(type P) *P
+type E2(type P) struct{ P }
+type E3(type P) struct{ *P }
+
+type T0 /* ERROR illegal cycle */ struct {
+        _ E0(T0)
+}
+
+type T0_ /* ERROR illegal cycle */ struct {
+        (E0(T0_))
+}
+
+type T1 struct {
+        _ E1(T1)
+}
+
+type T2 /* ERROR illegal cycle */ struct {
+        _ E2(T2)
+}
+
+type T3 struct {
+        _ E3(T3)
+}
+
+// some more complex cases
+
+type T4 /* ERROR illegal cycle */ struct {
+	_ E0(E2(T4))
+}
+
+type T5 struct {
+	_ E0(E2(E0(E1(E2([10]T5)))))
+}
+
+type T6 /* ERROR illegal cycle */ struct {
+	_ E0([10]E2(E0(E2(E2(T6)))))
+}
+
+type T7 struct {
+	_ E0([]E2(E0(E2(E2(T6)))))
+}