2 files changed, 39 insertions, 5 deletions
diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index 9c37028037..53259f4a9b 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -1193,3 +1193,29 @@ func TestMarshalerError(t *testing.T) {
 		})
 	}
 }
+
+type marshaledValue string
+
+func (v marshaledValue) MarshalJSON() ([]byte, error) {
+	return []byte(v), nil
+}
+
+func TestIssue63379(t *testing.T) {
+	for _, v := range []string{
+		"[]<",
+		"[]>",
+		"[]&",
+		"[]\u2028",
+		"[]\u2029",
+		"{}<",
+		"{}>",
+		"{}&",
+		"{}\u2028",
+		"{}\u2029",
+	} {
+		_, err := Marshal(marshaledValue(v))
+		if err == nil {
+			t.Errorf("expected error for %q", v)
+		}
+	}
+}
diff --git a/src/encoding/json/indent.go b/src/encoding/json/indent.go
index 26bb5d2e47..01bfdf65e7 100644
--- a/src/encoding/json/indent.go
+++ b/src/encoding/json/indent.go
@@ -53,29 +53,37 @@ func appendCompact(dst, src []byte, escape bool) ([]byte, error) {
 	start := 0
 	for i, c := range src {
 		if escape && (c == '<' || c == '>' || c == '&') {
-			dst = append(dst, src[start:i]...)
+			if start < i {
+				dst = append(dst, src[start:i]...)
+			}
 			dst = append(dst, '\\', 'u', '0', '0', hex[c>>4], hex[c&0xF])
 			start = i + 1
 		}
 		// Convert U+2028 and U+2029 (E2 80 A8 and E2 80 A9).
 		if escape && c == 0xE2 && i+2 < len(src) && src[i+1] == 0x80 && src[i+2]&^1 == 0xA8 {
-			dst = append(dst, src[start:i]...)
+			if start < i {
+				dst = append(dst, src[start:i]...)
+			}
 			dst = append(dst, '\\', 'u', '2', '0', '2', hex[src[i+2]&0xF])
-			start = i + len("\u2029")
+			start = i + 3
 		}
 		v := scan.step(scan, c)
 		if v >= scanSkipSpace {
 			if v == scanError {
 				break
 			}
-			dst = append(dst, src[start:i]...)
+			if start < i {
+				dst = append(dst, src[start:i]...)
+			}
 			start = i + 1
 		}
 	}
 	if scan.eof() == scanError {
 		return dst[:origLen], scan.err
 	}
-	dst = append(dst, src[start:]...)
+	if start < len(src) {
+		dst = append(dst, src[start:]...)
+	}
 	return dst, nil
 }