exp/template: fix endless loop

No progress was made in indirect() if the reflect.Value
was an non-nil and non-empty interface.

R=r, r
CC=golang-dev
https://golang.org/cl/4810060

src/pkg/exp/template/exec.go

@@ -585,12 +585,12 @@ func (s *state) evalEmptyInterface(dot reflect.Value, n node) reflect.Value {
 // 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 {
+	for ; v.Kind() == reflect.Ptr || v.Kind() == reflect.Interface; v = v.Elem() {
 		if v.IsNil() {
 			return v, true
 		}
-		if v.Kind() == reflect.Ptr || v.NumMethod() == 0 {
-			v = v.Elem()
+		if v.Kind() == reflect.Interface && v.NumMethod() > 0 {
+			break
 		}
 	}
 	return v, false

src/pkg/exp/template/exec_test.go

@@ -75,6 +75,13 @@ var tVal = &T{
 	Tmpl:   Must(New("x").Parse("test template")), // "x" is the value of .X
 }
 
+// A non-empty interface.
+type I interface {
+	Method0() string
+}
+
+var iVal I = tVal
+
 // Helpers for creation.
 func newInt(n int) *int {
 	p := new(int)
@@ -344,6 +351,9 @@ var execTests = []execTest{
 	// Fixed bugs.
 	// Must separate dot and receiver; otherwise args are evaluated with dot set to variable.
 	{"bug0", "{{range .MSIone}}{{if $.Method1 .}}X{{end}}{{end}}", "X", tVal, true},
+	// Do not loop endlessly in indirect for non-empty interfaces.
+	// The bug appears with *interface only; this is supposed to fail (cannot invoke Method0), but terminate.
+	{"bug1", "{{.Method0}}", "", &iVal, false},
 }
 
 func zeroArgs() string {