[dev.go2go] go/go2go: error, not panic, for local parameterized type

Fixes #39643

Change-Id: I45aaaf8818d4d383d1514891b5cc9bc68926ad97
Reviewed-on: https://go-review.googlesource.com/c/go/+/238619
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/go2go/rewrite.go

@@ -416,7 +416,8 @@ func (t *translator) translate(file *ast.File) {
 func (t *translator) translateTypeSpec(ps *ast.Spec) {
 	ts := (*ps).(*ast.TypeSpec)
 	if ts.TParams != nil {
-		panic("parameterized type")
+		t.err = fmt.Errorf("%s: go2go tool does not support parameterized type here", t.fset.Position((*ps).Pos()))
+		return
 	}
 	t.translateExpr(&ts.Type)
 }

test/gen/err004.go2

@@ -0,0 +1,11 @@
+// errorcheck
+
+// 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.
+
+package p
+
+func F() {
+     type t(type T) T // ERROR "parameterized type"
+}