encoding/json: different decision on tags and shadowing

If there are no tags, the rules are the same as before.
If there is a tagged field, choose it if there is exactly one
at the top level of all fields.
More tests. The old tests were clearly inadequate, since
they all pass as is. The new tests only work with the new code.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/8617044

src/pkg/encoding/json/encode.go

@@ -666,19 +666,17 @@ func typeFields(t reflect.Type) []field {
 		// Find the sequence of fields with the name of this first field.
 		fi := fields[i]
 		name := fi.name
-		hasTags := fi.tag
 		for advance = 1; i+advance < len(fields); advance++ {
 			fj := fields[i+advance]
 			if fj.name != name {
 				break
 			}
-			hasTags = hasTags || fj.tag
 		}
 		if advance == 1 { // Only one field with this name
 			out = append(out, fi)
 			continue
 		}
-		dominant, ok := dominantField(fields[i:i+advance], hasTags)
+		dominant, ok := dominantField(fields[i : i+advance])
 		if ok {
 			out = append(out, dominant)
 		}
@@ -696,23 +694,33 @@ func typeFields(t reflect.Type) []field {
 // JSON tags. If there are multiple top-level fields, the boolean
 // will be false: This condition is an error in Go and we skip all
 // the fields.
-func dominantField(fields []field, hasTags bool) (field, bool) {
-	if hasTags {
-		// If there's a tag, it gets promoted, so delete all fields without tags.
-		var j int
-		for i := 0; i < len(fields); i++ {
-			if fields[i].tag {
-				fields[j] = fields[i]
-				j++
+func dominantField(fields []field) (field, bool) {
+	// The fields are sorted in increasing index-length order. The winner
+	// must therefore be one with the shortest index length. Drop all
+	// longer entries, which is easy: just truncate the slice.
+	length := len(fields[0].index)
+	tagged := -1 // Index of first tagged field.
+	for i, f := range fields {
+		if len(f.index) > length {
+			fields = fields[:i]
+			break
+		}
+		if f.tag {
+			if tagged >= 0 {
+				// Multiple tagged fields at the same level: conflict.
+				// Return no field.
+				return field{}, false
 			}
+			tagged = i
 		}
-		fields = fields[:j]
 	}
-	// The fields are sorted in increasing index-length order. The first entry
-	// therefore wins, unless the second entry is of the same length. If that
-	// is true, then there is a conflict (two fields named "X" at the same level)
-	// and we have no fields.
-	if len(fields) > 1 && len(fields[0].index) == len(fields[1].index) {
+	if tagged >= 0 {
+		return fields[tagged], true
+	}
+	// All remaining fields have the same length. If there's more than one,
+	// we have a conflict (two fields named "X" at the same level) and we
+	// return no field.
+	if len(fields) > 1 {
 		return field{}, false
 	}
 	return fields[0], true

src/pkg/encoding/json/encode_test.go

@@ -221,7 +221,7 @@ type BugC struct {
 }
 
 // Legal Go: We never use the repeated embedded field (S).
-type BugD struct {
+type BugX struct {
 	A int
 	BugA
 	BugB
@@ -243,7 +243,7 @@ func TestEmbeddedBug(t *testing.T) {
 		t.Fatalf("Marshal: got %s want %s", got, want)
 	}
 	// Now check that the duplicate field, S, does not appear.
-	x := BugD{
+	x := BugX{
 		A: 23,
 	}
 	b, err = Marshal(x)
@@ -256,3 +256,57 @@ func TestEmbeddedBug(t *testing.T) {
 		t.Fatalf("Marshal: got %s want %s", got, want)
 	}
 }
+
+type BugD struct { // Same as BugA after tagging.
+	XXX string `json:"S"`
+}
+
+// BugD's tagged S field should dominate BugA's.
+type BugY struct {
+	BugA
+	BugD
+}
+
+// Test that a field with a tag dominates untagged fields.
+func TestTaggedFieldDominates(t *testing.T) {
+	v := BugY{
+		BugA{"BugA"},
+		BugD{"BugD"},
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{"S":"BugD"}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}
+
+// There are no tags here, so S should not appear.
+type BugZ struct {
+	BugA
+	BugC
+	BugY // Contains a tagged S field through BugD; should not dominate.
+}
+
+func TestDuplicatedFieldDisappears(t *testing.T) {
+	v := BugZ{
+		BugA{"BugA"},
+		BugC{"BugC"},
+		BugY{
+			BugA{"nested BugA"},
+			BugD{"nested BugD"},
+		},
+	}
+	b, err := Marshal(v)
+	if err != nil {
+		t.Fatal("Marshal:", err)
+	}
+	want := `{}`
+	got := string(b)
+	if got != want {
+		t.Fatalf("Marshal: got %s want %s", got, want)
+	}
+}