cmd/compile/internal/types2: add a test for argument error unwrapping

This CL is a clean port of CL 351338 from go/types to types2.

Change-Id: I7fd0e5a447bf51cb359e71731c2f9b95e3960da6
Reviewed-on: https://go-review.googlesource.com/c/go/+/364536
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -2014,6 +2014,20 @@ func TestInstantiateErrors(t *testing.T) {
 	}
 }
 
+func TestArgumentErrorUnwrapping(t *testing.T) {
+	var err error = &ArgumentError{
+		Index: 1,
+		Err:   Error{Msg: "test"},
+	}
+	var e Error
+	if !errors.As(err, &e) {
+		t.Fatalf("error %v does not wrap types.Error", err)
+	}
+	if e.Msg != "test" {
+		t.Errorf("e.Msg = %q, want %q", e.Msg, "test")
+	}
+}
+
 func TestInstanceIdentity(t *testing.T) {
 	imports := make(testImporter)
 	conf := Config{Importer: imports}