exp/template: make index function on maps return zero value when key not present.

R=r
CC=golang-dev
https://golang.org/cl/4808065

src/pkg/exp/template/exec_test.go

@@ -297,7 +297,7 @@ 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, false},
+	{"map[NO]", "{{index .MSI `XXX`}}", "", tVal, true},
 	{"map[WRONG]", "{{index .MSI 10}}", "", tVal, false},
 	{"double index", "{{index .SMSI 1 `eleven`}}", "11", tVal, true},
 

src/pkg/exp/template/funcs.go

@@ -109,9 +109,10 @@ func index(item interface{}, indices ...interface{}) (interface{}, os.Error) {
 			if !index.Type().AssignableTo(v.Type().Key()) {
 				return nil, fmt.Errorf("%s is not index type for %s", index.Type(), v.Type())
 			}
-			v = v.MapIndex(index)
-			if !v.IsValid() {
-				return nil, fmt.Errorf("index %v not present in map", index.Interface())
+			if x := v.MapIndex(index); x.IsValid() {
+				v = x
+			} else {
+				v = reflect.Zero(v.Type().Key())
 			}
 		default:
 			return nil, fmt.Errorf("can't index item of type %s", index.Type())