text/template: type-check chained node as argument

Was just a missing case (literally) in the type checker.

Fixes #8473.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews
https://golang.org/cl/142460043

src/text/template/exec.go

@@ -636,6 +636,8 @@ func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) refle
 		return s.validateType(s.evalPipeline(dot, arg), typ)
 	case *parse.IdentifierNode:
 		return s.evalFunction(dot, arg, arg, nil, zero)
+	case *parse.ChainNode:
+		return s.validateType(s.evalChainNode(dot, arg, nil, zero), typ)
 	}
 	switch typ.Kind() {
 	case reflect.Bool:

src/text/template/exec_test.go

@@ -176,6 +176,12 @@ func (t *T) Method3(v interface{}) string {
 	return fmt.Sprintf("Method3: %v", v)
 }
 
+func (t *T) Copy() *T {
+	n := new(T)
+	*n = *t
+	return n
+}
+
 func (t *T) MAdd(a int, b []int) []int {
 	v := make([]int, len(b))
 	for i, x := range b {
@@ -519,6 +525,8 @@ var execTests = []execTest{
 	{"bug12xE", "{{printf `%T` 0xEE}}", "int", T{}, true},
 	{"bug12Xe", "{{printf `%T` 0Xef}}", "int", T{}, true},
 	{"bug12XE", "{{printf `%T` 0XEE}}", "int", T{}, true},
+	// Chained nodes did not work as arguments. Issue 8473.
+	{"bug13", "{{print (.Copy).I}}", "17", tVal, true},
 }
 
 func zeroArgs() string {