encoding/json: don't match field name if a JSON struct tag is present.

Fixes #3566.

R=rsc
CC=golang-dev
https://golang.org/cl/6139048

src/pkg/encoding/json/decode.go

@@ -504,10 +504,15 @@ func (d *decodeState) object(v reflect.Value) {
 				}
 				// First, tag match
 				tagName, _ := parseTag(tag)
-				if tagName == key {
-					f = sf
-					ok = true
-					break // no better match possible
+				if tagName != "" {
+					if tagName == key {
+						f = sf
+						ok = true
+						break // no better match possible
+					}
+					// There was a tag, but it didn't match.
+					// Ignore field names.
+					continue
 				}
 				// Second, exact field name match
 				if sf.Name == key {

src/pkg/encoding/json/decode_test.go

@@ -18,6 +18,10 @@ type T struct {
 	Z int `json:"-"`
 }
 
+type U struct {
+	Alphabet string `json:"alpha"`
+}
+
 type tx struct {
 	x int
 }
@@ -72,6 +76,10 @@ var unmarshalTests = []unmarshalTest{
 	// Z has a "-" tag.
 	{`{"Y": 1, "Z": 2}`, new(T), T{Y: 1}, nil},
 
+	{`{"alpha": "abc", "alphabet": "xyz"}`, new(U), U{Alphabet: "abc"}, nil},
+	{`{"alpha": "abc"}`, new(U), U{Alphabet: "abc"}, nil},
+	{`{"alphabet": "xyz"}`, new(U), U{}, nil},
+
 	// syntax errors
 	{`{"X": "foo", "Y"}`, nil, nil, &SyntaxError{"invalid character '}' after object key", 17}},
 	{`[1, 2, 3+]`, nil, nil, &SyntaxError{"invalid character '+' after array element", 9}},