html/template: add Clone and AddParseTree. Make text/template's Clone

return (*Template, error), not just *Template.

Fixes #2757.

R=r
CC=golang-dev
https://golang.org/cl/5665044

2 files changed, 5 insertions, 5 deletions

diff --git a/src/pkg/text/template/multi_test.go b/src/pkg/text/template/multi_test.go
index 274f5ef147..f205e6be1b 100644
--- a/src/pkg/text/template/multi_test.go
+++ b/src/pkg/text/template/multi_test.go
@@ -193,7 +193,7 @@ func TestClone(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	clone := root.Clone()
+	clone := Must(root.Clone())
 	// Add variants to both.
 	_, err = root.Parse(cloneText3)
 	if err != nil {
diff --git a/src/pkg/text/template/template.go b/src/pkg/text/template/template.go
index 87e39d3af7..7494f9d8c4 100644
--- a/src/pkg/text/template/template.go
+++ b/src/pkg/text/template/template.go
@@ -69,9 +69,9 @@ func (t *Template) init() {
 // templates. The actual representation is not copied, but the name space of
 // associated templates is, so further calls to Parse in the copy will add
 // templates to the copy but not to the original. Clone can be used to prepare
-// common templates and use them with variant definitions for other templates by
-// adding the variants after the clone is made.
-func (t *Template) Clone() *Template {
+// common templates and use them with variant definitions for other templates
+// by adding the variants after the clone is made.
+func (t *Template) Clone() (*Template, error) {
 	nt := t.copy(nil)
 	nt.init()
 	nt.tmpl[t.name] = nt
@@ -89,7 +89,7 @@ func (t *Template) Clone() *Template {
 	for k, v := range t.execFuncs {
 		nt.execFuncs[k] = v
 	}
-	return nt
+	return nt, nil
 }
 
 // copy returns a shallow copy of t, with common set to the argument.