text/template: detect unmatched else at parsing time

An unmatched {{else}} should trigger a parsing error.

The top level parser is able to issue an error in case
of unmatched {{end}}. It does it a posteriori (i.e. after having
parsed the action).

Extend this behavior to also check for unmatched {{else}}

Fixes #10611

Change-Id: I1d4f433cc64e11bea5f4d61419ccc707ac01bb1d
Reviewed-on: https://go-review.googlesource.com/9620
Reviewed-by: Rob Pike <r@golang.org>

src/text/template/parse/parse.go

@@ -288,11 +288,12 @@ func (t *Tree) parse(treeSet map[string]*Tree) (next Node) {
 			}
 			t.backup2(delim)
 		}
-		n := t.textOrAction()
+		switch n := t.textOrAction(); n.Type() {
-		if n.Type() == nodeEnd {
+		case nodeEnd, nodeElse:
 			t.errorf("unexpected %s", n)
+		default:
+			t.Root.append(n)
 		}
-		t.Root.append(n)
 	}
 	return nil
 }

src/text/template/parse/parse_test.go

@@ -230,6 +230,9 @@ var parseTests = []parseTest{
 	// Errors.
 	{"unclosed action", "hello{{range", hasError, ""},
 	{"unmatched end", "{{end}}", hasError, ""},
+	{"unmatched else", "{{else}}", hasError, ""},
+	{"unmatched else after if", "{{if .X}}hello{{end}}{{else}}", hasError, ""},
+	{"multiple else", "{{if .X}}1{{else}}2{{else}}3{{end}}", hasError, ""},
 	{"missing end", "hello{{range .x}}", hasError, ""},
 	{"missing end after else", "hello{{range .x}}{{else}}", hasError, ""},
 	{"undefined function", "hello{{undefined}}", hasError, ""},