exp/template: do not take address of interface variable to find methods.

Also allow struct values as "with" targets.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/4809086

src/pkg/exp/template/exec.go

@@ -179,6 +179,8 @@ func isTrue(val reflect.Value) (truth, ok bool) {
 		truth = val.Float() != 0
 	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
 		truth = val.Uint() != 0
+	case reflect.Struct:
+		truth = true // Struct values are always true.
 	default:
 		return
 	}
@@ -377,10 +379,10 @@ func (s *state) evalField(dot reflect.Value, fieldName string, args []parse.Node
 	}
 	typ := receiver.Type()
 	receiver, _ = indirect(receiver)
-	// Need to get to a value of type *T to guarantee we see all
-	// methods of T and *T.
+	// Unless it's an interface, need to get to a value of type *T to guarantee
+	// we see all methods of T and *T.
 	ptr := receiver
-	if ptr.CanAddr() {
+	if ptr.Kind() != reflect.Interface && ptr.CanAddr() {
 		ptr = ptr.Addr()
 	}
 	if method, ok := methodByName(ptr, fieldName); ok {

src/pkg/exp/template/exec_test.go

@@ -43,6 +43,8 @@ type T struct {
 	Empty2 interface{}
 	Empty3 interface{}
 	Empty4 interface{}
+	// Non-empty interface.
+	NonEmptyInterface I
 	// Pointers
 	PI  *int
 	PSI *[]int
@@ -73,6 +75,7 @@ var tVal = &T{
 	Empty2:            "empty2",
 	Empty3:            []int{7, 8},
 	Empty4:            &U{"UinEmpty"},
+	NonEmptyInterface: new(T),
 	PI:                newInt(23),
 	PSI:               newIntSlice(21, 22, 23),
 	Tmpl:              Must(New("x").Parse("test template")), // "x" is the value of .X
@@ -358,8 +361,12 @@ var execTests = []execTest{
 	// Must separate dot and receiver; otherwise args are evaluated with dot set to variable.
 	{"bug0", "{{range .MSIone}}{{if $.Method1 .}}X{{end}}{{end}}", "X", tVal, true},
 	// Do not loop endlessly in indirect for non-empty interfaces.
-	// The bug appears with *interface only; this is supposed to fail (cannot invoke Method0), but terminate.
-	{"bug1", "{{.Method0}}", "", &iVal, false},
+	// The bug appears with *interface only; looped forever.
+	{"bug1", "{{.Method0}}", "M0", &iVal, true},
+	// Was taking address of interface field, so method set was empty.
+	{"bug2", "{{$.NonEmptyInterface.Method0}}", "M0", tVal, true},
+	// Struct values were not legal in with - mere oversight.
+	{"bug4", "{{with $}}{{.Method0}}{{end}}", "M0", tVal, true},
 }
 
 func zeroArgs() string {