[regexp] Handle unmatched groups in callable replacers

test/mjsunit/harmony/regexp-named-captures.js

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Flags: --harmony-regexp-named-captures
 
 // Malformed named captures.
 assertThrows("/(?<>a)/u");  // Empty name.
 assertThrows("/(?<aa)/u");  // Unterminated name.
 assertThrows("/(?<42a>a)/u");  // Name starting with digits.
@@ skipping 90 matching lines @@
 {
   let result = "abcd".replace(/(.)(.)/u, (match, fst, snd, offset, str) => {
     assertEquals("ab", match);
     assertEquals("a", fst);
     assertEquals("b", snd);
     assertEquals(0, offset);
     assertEquals("abcd", str);
     return `${snd}${fst}`;
   });
   assertEquals("bacd", result);
+
+  assertEquals("undefinedbcd", "abcd".replace(/(.)|(.)/u,
+      (match, fst, snd, offset, str) => snd));
 }
 
 // @@replace with a callable replacement argument (global, named captures).
 {
   let i = 0;
   let result = "abcd".replace(/(?<fst>.)(?<snd>.)/gu,
       (match, fst, snd, offset, str, groups) => {
     if (i == 0) {
       assertEquals("ab", match);
       assertEquals("a", groups.fst);
@@ skipping 10 matching lines @@
       assertEquals("d", snd);
       assertEquals(2, offset);
       assertEquals("abcd", str);
     } else {
       assertUnreachable();
     }
     i++;
     return `${groups.snd}${groups.fst}`;
   });
   assertEquals("badc", result);
+
+  assertEquals("undefinedundefinedundefinedundefined",
+      "abcd".replace(/(?<fst>.)|(?<snd>.)/gu,
+            (match, fst, snd, offset, str, groups) => groups.snd));
 }
 
 // @@replace with a callable replacement argument (non-global, named captures).
 {
   let result = "abcd".replace(/(?<fst>.)(?<snd>.)/u,
       (match, fst, snd, offset, str, groups) => {
     assertEquals("ab", match);
     assertEquals("a", groups.fst);
     assertEquals("b", groups.snd);
     assertEquals("a", fst);
     assertEquals("b", snd);
     assertEquals(0, offset);
     assertEquals("abcd", str);
     return `${groups.snd}${groups.fst}`;
   });
   assertEquals("bacd", result);
+
+  assertEquals("undefinedbcd",
+      "abcd".replace(/(?<fst>.)|(?<snd>.)/u,
+            (match, fst, snd, offset, str, groups) => groups.snd));
 }
 
 function toSlowMode(re) {
   re.exec = (str) => RegExp.prototype.exec.call(re, str);
   return re;
 }
 
 // @@replace with a callable replacement argument (slow, global,
 // named captures).
 {
@@ skipping 16 matching lines @@
       assertEquals("d", snd);
       assertEquals(2, offset);
       assertEquals("abcd", str);
     } else {
       assertUnreachable();
     }
     i++;
     return `${groups.snd}${groups.fst}`;
   });
   assertEquals("badc", result);
+
+  assertEquals("undefinedundefinedundefinedundefined",
+      "abcd".replace(toSlowMode(/(?<fst>.)|(?<snd>.)/gu),
+            (match, fst, snd, offset, str, groups) => groups.snd));
 }
 
 // @@replace with a callable replacement argument (slow, non-global,
 // named captures).
 {
   let re = toSlowMode(/(?<fst>.)(?<snd>.)/u);
   let result = "abcd".replace(re, (match, fst, snd, offset, str, groups) => {
     assertEquals("ab", match);
     assertEquals("a", groups.fst);
     assertEquals("b", groups.snd);
     assertEquals("a", fst);
     assertEquals("b", snd);
     assertEquals(0, offset);
     assertEquals("abcd", str);
     return `${groups.snd}${groups.fst}`;
   });
   assertEquals("bacd", result);
+
+  assertEquals("undefinedbcd",
+      "abcd".replace(toSlowMode(/(?<fst>.)|(?<snd>.)/u),
+            (match, fst, snd, offset, str, groups) => groups.snd));
 }
 
 // @@replace with a string replacement argument (no named captures).
 {
   let re = /(.)(.)/u;
   assertEquals("$<snd>$<fst>cd", "abcd".replace(re, "$<snd>$<fst>"));
   assertEquals("bacd", "abcd".replace(re, "$2$1"));
   assertEquals("$<sndcd", "abcd".replace(re, "$<snd"));
   assertEquals("$<42a>cd", "abcd".replace(re, "$<42$1>"));
   assertEquals("$<thd>cd", "abcd".replace(re, "$<thd>"));
@@ skipping 37 matching lines @@
 // named captures).
 {
   let re = toSlowMode(/(?<fst>.)(?<snd>.)/u);
   assertEquals("bacd", "abcd".replace(re, "$<snd>$<fst>"));
   assertEquals("bacd", "abcd".replace(re, "$2$1"));
   assertThrows(() => "abcd".replace(re, "$<snd"), SyntaxError);
   assertEquals("cd", "abcd".replace(re, "$<42$1>"));
   assertEquals("cd", "abcd".replace(re, "$<thd>"));
   assertEquals("cd", "abcd".replace(re, "$<$1>"));
 }