go/types, types2: add test case for issue for coverage

The specific error doesn't occur anymore.
Add a test to prevent regressions.

For #50729.

Change-Id: Ibf6ef6009b3d226b4f345b5a5657939915f19633
Reviewed-on: https://go-review.googlesource.com/c/go/+/412235
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50729.go

@@ -0,0 +1,19 @@
+// Copyright 2022 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
+
+// version 1
+var x1 T1[B1]
+
+type T1[_ any] struct{}
+type A1 T1[B1]
+type B1 = T1[A1]
+
+// version 2
+type T2[_ any] struct{}
+type A2 T2[B2]
+type B2 = T2[A2]
+
+var x2 T2[B2]

src/go/types/testdata/fixedbugs/issue50729.go

@@ -0,0 +1,19 @@
+// Copyright 2022 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
+
+// version 1
+var x1 T1[B1]
+
+type T1[_ any] struct{}
+type A1 T1[B1]
+type B1 = T1[A1]
+
+// version 2
+type T2[_ any] struct{}
+type A2 T2[B2]
+type B2 = T2[A2]
+
+var x2 T2[B2]