text/template: resolve non-empty interface

Read what a non-empty interface points to.

The deleted lines were added in https://codereview.appspot.com/4810060/,
which attempted to break an infinite loop. That was a long time ago.
If I just delete these lines with current codebase, the test "bug1"
(added in that CL) does not fail.

All new tests fail without this fix.

Fixes #12924

Change-Id: I9370ca44facd6af3019850aa065b936e5a482d37
Reviewed-on: https://go-review.googlesource.com/15809Reviewed-by: Andrew Gerrand <adg@golang.org>

src/text/template/exec.go

@@ -829,16 +829,11 @@ func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Valu
 }
 
 // indirect returns the item at the end of indirection, and a bool to indicate if it's nil.
-// We indirect through pointers and empty interfaces (only) because
-// non-empty interfaces have methods we might need.
 func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
 	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
 		if v.IsNil() {
 			return v, true
 		}
-		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
-			break
-		}
 	}
 	return v, false
 }

src/text/template/exec_test.go

@@ -51,8 +51,9 @@ type T struct {
 	Empty2 interface{}
 	Empty3 interface{}
 	Empty4 interface{}
-	// Non-empty interface.
+	// Non-empty interfaces.
 	NonEmptyInterface    I
+	NonEmptyInterfacePtS *I
 	// Stringer.
 	Str fmt.Stringer
 	Err error
@@ -73,6 +74,12 @@ type T struct {
 	unexported int
 }
 
+type S []string
+
+func (S) Method0() string {
+	return "M0"
+}
+
 type U struct {
 	V string
 }
@@ -99,6 +106,8 @@ func (w *W) Error() string {
 	return fmt.Sprintf("[%d]", w.k)
 }
 
+var siVal = I(S{"a", "b"})
+
 var tVal = &T{
 	True:   true,
 	I:      17,
@@ -123,7 +132,8 @@ var tVal = &T{
 	Empty2:               "empty2",
 	Empty3:               []int{7, 8},
 	Empty4:               &U{"UinEmpty"},
-	NonEmptyInterface: new(T),
+	NonEmptyInterface:    &T{X: "x"},
+	NonEmptyInterfacePtS: &siVal,
 	Str:                  bytes.NewBuffer([]byte("foozle")),
 	Err:                  errors.New("erroozle"),
 	PI:                   newInt(23),
@@ -550,6 +560,11 @@ var execTests = []execTest{
 	{"bug16i", "{{\"aaa\"|oneArg}}", "oneArg=aaa", tVal, true},
 	{"bug16j", "{{1+2i|printf \"%v\"}}", "(1+2i)", tVal, true},
 	{"bug16k", "{{\"aaa\"|printf }}", "aaa", tVal, true},
+	{"bug17a", "{{.NonEmptyInterface.X}}", "x", tVal, true},
+	{"bug17b", "-{{.NonEmptyInterface.Method1 1234}}-", "-1234-", tVal, true},
+	{"bug17c", "{{len .NonEmptyInterfacePtS}}", "2", tVal, true},
+	{"bug17d", "{{index .NonEmptyInterfacePtS 0}}", "a", tVal, true},
+	{"bug17e", "{{range .NonEmptyInterfacePtS}}-{{.}}-{{end}}", "-a--b-", tVal, true},
 }
 
 func zeroArgs() string {