exp/template: fix implementation of printValue.

R=r
CC=golang-dev
https://golang.org/cl/4878042

src/pkg/exp/template/exec.go

@@ -620,19 +620,23 @@ func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
 // printValue writes the textual representation of the value to the output of
 // the template.
 func (s *state) printValue(n parse.Node, v reflect.Value) {
+	if v.Kind() == reflect.Ptr {
+		v, _ = indirect(v) // fmt.Fprint handles nil.
+	}
 	if !v.IsValid() {
 		fmt.Fprint(s.wr, "<no value>")
 		return
 	}
+
+	if !v.Type().Implements(fmtStringerType) {
+		if v.CanAddr() && reflect.PtrTo(v.Type()).Implements(fmtStringerType) {
+			v = v.Addr()
+		} else {
 			switch v.Kind() {
-	case reflect.Ptr:
-		v, _ = indirect(v) // fmt.Fprint handles nil.
-	case reflect.Chan, reflect.Func, reflect.Interface:
+			case reflect.Chan, reflect.Func:
 				s.errorf("can't print %s of type %s", n, v.Type())
 			}
-	// If it's a value but the pointer implements Stringer, use the pointer.
-	if v.Kind() != reflect.Ptr && v.CanAddr() && reflect.PtrTo(v.Type()).Implements(fmtStringerType) {
-		v = v.Addr()
+		}
 	}
 	fmt.Fprint(s.wr, v.Interface())
 }

src/pkg/exp/template/exec_test.go

@@ -48,6 +48,8 @@ type T struct {
 	Empty4 interface{}
 	// Non-empty interface.
 	NonEmptyInterface I
+	// Stringer.
+	Str fmt.Stringer
 	// Pointers
 	PI  *int
 	PSI *[]int
@@ -92,6 +94,7 @@ var tVal = &T{
 	Empty3:            []int{7, 8},
 	Empty4:            &U{"UinEmpty"},
 	NonEmptyInterface: new(T),
+	Str:               os.NewError("foozle"),
 	PI:                newInt(23),
 	PSI:               newIntSlice(21, 22, 23),
 	Tmpl:              Must(New("x").Parse("test template")), // "x" is the value of .X
@@ -396,6 +399,8 @@ var execTests = []execTest{
 	{"bug3", "{{with $}}{{.Method0}}{{end}}", "M0", tVal, true},
 	// Nil interface values in if.
 	{"bug4", "{{if .Empty0}}non-nil{{else}}nil{{end}}", "nil", tVal, true},
+	// Stringer.
+	{"bug5", "{{.Str}}", "foozle", tVal, true},
 }
 
 func zeroArgs() string {