[dev.go2go] go/go2go: add package qualifier when needed

Fixes #39842

Change-Id: I5046f1bdd7dbc6d1e17cf3835826933aaadf1581
Reviewed-on: https://go-review.googlesource.com/c/go/+/239711
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/go2go/rewrite.go

@@ -648,6 +648,7 @@ func (t *translator) translateExpr(pe *ast.Expr) {
 	}
 	switch e := (*pe).(type) {
 	case *ast.Ident:
+		t.translateIdent(pe)
 	case *ast.Ellipsis:
 		t.translateExpr(&e.Elt)
 	case *ast.BasicLit:
@@ -715,6 +716,38 @@ func (t *translator) translateExpr(pe *ast.Expr) {
 	}
 }
 
+// translateIdent translates a simple identifier from Go with
+// contracts to Go 1. These are usually fine as is, but a reference
+// to a non-generic name in another package may need a package qualifier.
+func (t *translator) translateIdent(pe *ast.Expr) {
+	e := (*pe).(*ast.Ident)
+	obj := t.importer.info.ObjectOf(e)
+	if obj == nil {
+		return
+	}
+	if named, ok := obj.Type().(*types.Named); ok && len(named.TParams()) > 0 {
+		// A generic function that will be instantiated locally.
+		return
+	}
+	ipkg := obj.Pkg()
+	if ipkg == nil || ipkg == t.tpkg {
+		// We don't need a package qualifier if it's defined
+		// in the current package.
+		return
+	}
+	if obj.Parent() != ipkg.Scope() {
+		// We only need a package qualifier if it's defined in
+		// package scope.
+		return
+	}
+
+	// Add package qualifier.
+	*pe = &ast.SelectorExpr{
+		X:   ast.NewIdent(ipkg.Name()),
+		Sel: e,
+	}
+}
+
 // translateSelectorExpr translates a selector expression
 // from Go with contracts to Go 1.
 func (t *translator) translateSelectorExpr(pe *ast.Expr) {

test/gen/g037.dir/a.go2

@@ -0,0 +1,11 @@
+// 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 a
+
+func F() {}
+
+func G(type T)() {
+	F()
+}

test/gen/g037.dir/b.go2

@@ -0,0 +1,11 @@
+// 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 main
+
+import "./a"
+
+func main() {
+	a.G(int)()
+}

test/gen/g037.go2

@@ -0,0 +1,7 @@
+// rundir
+
+// 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 ignored