text/template: fix bug in map indexing

If the key is not present, return value of the type of the element
not the type of the key. Also fix a test that should have caught this case.

Fixes #3850.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6405078

src/pkg/text/template/exec_test.go

@@ -390,7 +390,7 @@ var execTests = []execTest{
 	{"slice[WRONG]", "{{index .SI `hello`}}", "", tVal, false},
 	{"map[one]", "{{index .MSI `one`}}", "1", tVal, true},
 	{"map[two]", "{{index .MSI `two`}}", "2", tVal, true},
-	{"map[NO]", "{{index .MSI `XXX`}}", "", tVal, true},
+	{"map[NO]", "{{index .MSI `XXX`}}", "0", tVal, true},
 	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
 	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
 

src/pkg/text/template/funcs.go

@@ -128,7 +128,7 @@ func index(item interface{}, indices ...interface{}) (interface{}, error) {
 			if x := v.MapIndex(index); x.IsValid() {
 				v = x
 			} else {
-				v = reflect.Zero(v.Type().Key())
+				v = reflect.Zero(v.Type().Elem())
 			}
 		default:
 			return nil, fmt.Errorf("can't index item of type %s", index.Type())