template: indirect or dereference function arguments if necessary to match the type of the formal.

Fixes #2235

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4967056

src/pkg/template/exec.go

@@ -506,7 +506,18 @@ func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Valu
 		s.errorf("invalid value; expected %s", typ)
 	}
 	if !value.Type().AssignableTo(typ) {
-		s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
+		// Does one dereference or indirection work? We could do more, as we
+		// do with method receivers, but that gets messy and method receivers
+		// are much more constrained, so it makes more sense there than here.
+		// Besides, one is almost always all you need.
+		switch {
+		case value.Kind() == reflect.Ptr && value.Elem().Type().AssignableTo(typ):
+			value = value.Elem()
+		case reflect.PtrTo(value.Type()).AssignableTo(typ) && value.CanAddr():
+			value = value.Addr()
+		default:
+			s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
+		}
 	}
 	return value
 }

src/pkg/template/exec_test.go

@@ -416,6 +416,11 @@ var execTests = []execTest{
 	{"bug4", "{{if .Empty0}}non-nil{{else}}nil{{end}}", "nil", tVal, true},
 	// Stringer.
 	{"bug5", "{{.Str}}", "foozle", tVal, true},
+	// Args need to be indirected and dereferenced sometimes.
+	{"bug6a", "{{vfunc .V0 .V1}}", "vfunc", tVal, true},
+	{"bug6b", "{{vfunc .V0 .V0}}", "vfunc", tVal, true},
+	{"bug6c", "{{vfunc .V1 .V0}}", "vfunc", tVal, true},
+	{"bug6d", "{{vfunc .V1 .V1}}", "vfunc", tVal, true},
 }
 
 func zeroArgs() string {
@@ -441,12 +446,18 @@ func count(n int) chan string {
 	return c
 }
 
+// vfunc takes a *V and a V
+func vfunc(V, *V) string {
+	return "vfunc"
+}
+
 func testExecute(execTests []execTest, set *Set, t *testing.T) {
 	b := new(bytes.Buffer)
 	funcs := FuncMap{
 		"count":    count,
 		"oneArg":   oneArg,
 		"typeOf":   typeOf,
+		"vfunc":    vfunc,
 		"zeroArgs": zeroArgs,
 	}
 	for _, test := range execTests {