text/template: fix handing of nil arguments to functions

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5494070

src/pkg/text/template/exec.go

@@ -497,7 +497,13 @@ func (s *state) evalCall(dot, fun reflect.Value, name string, args []parse.Node,
 // validateType guarantees that the value is valid and assignable to the type.
 func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
 	if !value.IsValid() {
-		s.errorf("invalid value; expected %s", typ)
+		switch typ.Kind() {
+		case reflect.Interface, reflect.Ptr, reflect.Chan, reflect.Map, reflect.Slice, reflect.Func:
+			// An untyped nil interface{}. Accept as a proper nil value.
+			value = reflect.Zero(typ)
+		default:
+			s.errorf("invalid value; expected %s", typ)
+		}
 	}
 	if !value.Type().AssignableTo(typ) {
 		// Does one dereference or indirection work? We could do more, as we

src/pkg/text/template/exec_test.go

@@ -157,6 +157,10 @@ func (t *T) Method2(a uint16, b string) string {
 	return fmt.Sprintf("Method2: %d %s", a, b)
 }
 
+func (t *T) Method3(v interface{}) string {
+	return fmt.Sprintf("Method3: %v", v)
+}
+
 func (t *T) MAdd(a int, b []int) []int {
 	v := make([]int, len(b))
 	for i, x := range b {
@@ -293,6 +297,7 @@ var execTests = []execTest{
 	{".Method2(3, .X)", "-{{.Method2 3 .X}}-", "-Method2: 3 x-", tVal, true},
 	{".Method2(.U16, `str`)", "-{{.Method2 .U16 `str`}}-", "-Method2: 16 str-", tVal, true},
 	{".Method2(.U16, $x)", "{{if $x := .X}}-{{.Method2 .U16 $x}}{{end}}-", "-Method2: 16 x-", tVal, true},
+	{".Method3(nil)", "-{{.Method3 .MXI.unset}}-", "-Method3: <nil>-", tVal, true},
 	{"method on var", "{{if $x := .}}-{{$x.Method2 .U16 $x.X}}{{end}}-", "-Method2: 16 x-", tVal, true},
 	{"method on chained var",
 		"{{range .MSIone}}{{if $.U.TrueFalse $.True}}{{$.U.TrueFalse $.True}}{{else}}WRONG{{end}}{{end}}",
@@ -322,6 +327,8 @@ var execTests = []execTest{
 	{"if slice", "{{if .SI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
 	{"if emptymap", "{{if .MSIEmpty}}NON-EMPTY{{else}}EMPTY{{end}}", "EMPTY", tVal, true},
 	{"if map", "{{if .MSI}}NON-EMPTY{{else}}EMPTY{{end}}", "NON-EMPTY", tVal, true},
+	{"if map unset", "{{if .MXI.none}}NON-ZERO{{else}}ZERO{{end}}", "ZERO", tVal, true},
+	{"if map not unset", "{{if not .MXI.none}}ZERO{{else}}NON-ZERO{{end}}", "ZERO", tVal, true},
 	{"if $x with $y int", "{{if $x := true}}{{with $y := .I}}{{$x}},{{$y}}{{end}}{{end}}", "true,17", tVal, true},
 	{"if $x with $x int", "{{if $x := true}}{{with $x := .I}}{{$x}},{{end}}{{$x}}{{end}}", "17,true", tVal, true},