exp/template: find the String method by taking the address if we need to.

Also simplify nil handling in printing.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4869042

src/pkg/exp/template/exec.go

@@ -425,7 +425,8 @@ func methodByName(receiver reflect.Value, name string) (reflect.Value, bool) {
 }
 
 var (
-	osErrorType = reflect.TypeOf(new(os.Error)).Elem()
+	osErrorType     = reflect.TypeOf((*os.Error)(nil)).Elem()
+	fmtStringerType = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
 )
 
 // evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
@@ -625,13 +626,13 @@ func (s *state) printValue(n parse.Node, v reflect.Value) {
 	}
 	switch v.Kind() {
 	case reflect.Ptr:
-		var isNil bool
-		if v, isNil = indirect(v); isNil {
-			fmt.Fprint(s.wr, "<nil>")
-			return
-		}
+		v, _ = indirect(v) // fmt.Fprint handles nil.
 	case reflect.Chan, reflect.Func, reflect.Interface:
 		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

@@ -28,6 +28,9 @@ type T struct {
 	ComplexZero float64
 	// Nested structs.
 	U *U
+	// Struct with String method.
+	V0     V
+	V1, V2 *V
 	// Slices
 	SI      []int
 	SIEmpty []int
@@ -57,12 +60,25 @@ type U struct {
 	V string
 }
 
+type V struct {
+	j int
+}
+
+func (v *V) String() string {
+	if v == nil {
+		return "nilV"
+	}
+	return fmt.Sprintf("<%d>", v.j)
+}
+
 var tVal = &T{
 	True:   true,
 	I:      17,
 	U16:    16,
 	X:      "x",
 	U:      &U{"v"},
+	V0:     V{6666},
+	V1:     &V{7777}, // leave V2 as nil
 	SI:     []int{3, 4, 5},
 	SB:     []bool{true, false},
 	MSI:    map[string]int{"one": 1, "two": 2, "three": 3},
@@ -212,6 +228,11 @@ var execTests = []execTest{
 	{"$.U.V", "{{$.U.V}}", "v", tVal, true},
 	{"declare in action", "{{$x := $.U.V}}{{$x}}", "v", tVal, true},
 
+	// Type with String method.
+	{"V{6666}.String()", "-{{.V0}}-", "-<6666>-", tVal, true},
+	{"&V{7777}.String()", "-{{.V1}}-", "-<7777>-", tVal, true},
+	{"(*V)(nil).String()", "-{{.V2}}-", "-nilV-", tVal, true},
+
 	// Pointers.
 	{"*int", "{{.PI}}", "23", tVal, true},
 	{"*[]int", "{{.PSI}}", "[21 22 23]", tVal, true},