json: don't indirect before testing for custom unmarshaler

Fixes #1260.

R=gri
CC=golang-dev
https://golang.org/cl/2994041

src/pkg/json/decode.go

@@ -128,7 +128,9 @@ func (d *decodeState) unmarshal(v interface{}) (err os.Error) {
 	}
 
 	d.scan.reset()
-	d.value(pv.Elem())
+	// We decode rv not pv.Elem because the Unmarshaler interface
+	// test must be applied at the top level of the value.
+	d.value(rv)
 	return d.savedError
 }
 

src/pkg/json/decode_test.go

@@ -23,6 +23,24 @@ type tx struct {
 
 var txType = reflect.Typeof((*tx)(nil)).(*reflect.PtrType).Elem().(*reflect.StructType)
 
+// A type that can unmarshal itself.
+
+type unmarshaler struct {
+	T bool
+}
+
+func (u *unmarshaler) UnmarshalJSON(b []byte) os.Error {
+	*u = unmarshaler{true} // All we need to see that UnmarshalJson is called.
+	return nil
+}
+
+var (
+	um0, um1 unmarshaler // target2 of unmarshaling
+	ump      = &um1
+	umtrue   = unmarshaler{true}
+)
+
+
 type unmarshalTest struct {
 	in  string
 	ptr interface{}
@@ -56,6 +74,10 @@ var unmarshalTests = []unmarshalTest{
 	{pallValueCompact, new(All), pallValue, nil},
 	{pallValueIndent, new(*All), &pallValue, nil},
 	{pallValueCompact, new(*All), &pallValue, nil},
+
+	// unmarshal interface test
+	{`{"T":false}`, &um0, umtrue, nil}, // use "false" so test will fail if custom unmarshaler is not called
+	{`{"T":false}`, &ump, &umtrue, nil},
 }
 
 func TestMarshal(t *testing.T) {