go/printer: print parenthesized declarations if len(d.Specs) > 1

Parenthesized declaration must be printed if len(d.Specs) > 1 even if d.Lparen==token.NoPos. This happens if the node tree is created programmatically. Otherwise, all but the first specifications just silently disappear from the output.

Change-Id: I17ab24bb1cd56fe1e611199698535ca60a97f5ea
GitHub-Last-Rev: 2f168dc7ad4a29149685efc70f180987523271e4
GitHub-Pull-Request: golang/go#28533
Reviewed-on: https://go-review.googlesource.com/c/146657
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/printer/nodes.go

@@ -1537,7 +1537,7 @@ func (p *printer) genDecl(d *ast.GenDecl) {
 	p.setComment(d.Doc)
 	p.print(d.Pos(), d.Tok, blank)
 
-	if d.Lparen.IsValid() {
+	if d.Lparen.IsValid() || len(d.Specs) > 1 {
 		// group of parenthesized declarations
 		p.print(d.Lparen, token.LPAREN)
 		if n := len(d.Specs); n > 0 {

src/go/printer/printer_test.go

@@ -736,3 +736,35 @@ func TestIssue11151(t *testing.T) {
 		t.Errorf("%v\norig: %q\ngot : %q", err, src, got)
 	}
 }
+
+// If a declaration has multiple specifications, a parenthesized
+// declaration must be printed even if Lparen is token.NoPos.
+func TestParenthesizedDecl(t *testing.T) {
+	// a package with multiple specs in a single declaration
+	const src = "package p; var ( a float64; b int )"
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "", src, 0)
+
+	// print the original package
+	var buf bytes.Buffer
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	original := buf.String()
+
+	// now remove parentheses from the declaration
+	for i := 0; i != len(f.Decls); i++ {
+		f.Decls[i].(*ast.GenDecl).Lparen = token.NoPos
+	}
+	buf.Reset()
+	err = Fprint(&buf, fset, f)
+	if err != nil {
+		t.Fatal(err)
+	}
+	noparen := buf.String()
+
+	if noparen != original {
+		t.Errorf("got %q, want %q", noparen, original)
+	}
+}