go/printer: don't strip parens around *ast.Ellipsis in return field list

Change-Id: Ic6a6d87202f9b531a591dc0ab2ef5fb050251198

src/go/printer/nodes.go

@@ -447,9 +447,13 @@ func (p *printer) signature(sig *ast.FuncType) {
 		// res != nil
 		p.print(blank)
 		if n == 1 && res.List[0].Names == nil {
-			// single anonymous res; no ()'s
-			p.expr(stripParensAlways(res.List[0].Type))
-			return
+			// Parser permits ellipsis inside of a return field list, like: func A() (...int),
+			// removing them would cause a parse error.
+			if _, ok := res.List[0].Type.(*ast.Ellipsis); !ok {
+				// single anonymous res; no ()'s
+				p.expr(stripParensAlways(res.List[0].Type))
+				return
+			}
 		}
 		p.parameters(res, funcParam)
 	}

src/go/printer/printer_test.go

@@ -16,6 +16,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 )
@@ -863,3 +864,20 @@ func TestEmptyDecl(t *testing.T) { // issue 63566
 		}
 	}
 }
+
+func TestPrintParsableEllipsisInReturnFieldList(t *testing.T) {
+	const src = "package A\n\nfunc A() (...A)\n"
+
+	fset := token.NewFileSet()
+	f, err := parser.ParseFile(fset, "test.go", src, parser.SkipObjectResolution)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var out strings.Builder
+	Fprint(&out, fset, f)
+
+	if out.String() != src {
+		t.Fatalf("source = %q; printed as = %q want = %q", src, out.String(), src)
+	}
+}