text/template: fix Parse when called twice with empty text

Fixes #16156

Change-Id: I6989db4fd392583a2d490339cefc525b07c11b90
Reviewed-on: https://go-review.googlesource.com/24380Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Andrew Gerrand <adg@golang.org>

src/text/template/multi_test.go

@@ -349,3 +349,39 @@ func TestParse(t *testing.T) {
 		t.Fatalf("parsing test: %s", err)
 	}
 }
+
+func TestEmptyTemplate(t *testing.T) {
+	cases := []struct {
+		defn []string
+		in   string
+		want string
+	}{
+		{[]string{""}, "once", ""},
+		{[]string{"", ""}, "twice", ""},
+		{[]string{"{{.}}", "{{.}}"}, "twice", "twice"},
+		{[]string{"{{/* a comment */}}", "{{/* a comment */}}"}, "comment", ""},
+		{[]string{"{{.}}", ""}, "twice", ""},
+	}
+
+	for _, c := range cases {
+		root := New("root")
+
+		var (
+			m   *Template
+			err error
+		)
+		for _, d := range c.defn {
+			m, err = root.New(c.in).Parse(d)
+			if err != nil {
+				t.Fatal(err)
+			}
+		}
+		buf := &bytes.Buffer{}
+		if err := m.Execute(buf, c.in); err != nil {
+			t.Fatal(err)
+		}
+		if buf.String() != c.want {
+			t.Errorf("expected string %q: got %q", c.want, buf.String())
+		}
+	}
+}

src/text/template/template.go

@@ -208,7 +208,7 @@ func (t *Template) associate(new *Template, tree *parse.Tree) (bool, error) {
 	if new.common != t.common {
 		panic("internal error: associate not common")
 	}
-	if t.tmpl[new.name] != nil && parse.IsEmptyTree(tree.Root) {
+	if t.tmpl[new.name] != nil && parse.IsEmptyTree(tree.Root) && t.Tree != nil {
 		// If a template by that name exists,
 		// don't replace it with an empty template.
 		return false, nil