Normative: allow duplicate named capture groups

spec.html

@@ -34639,7 +34639,7 @@ <h1>Static Semantics: Early Errors</h1>
             It is a Syntax Error if CountLeftCapturingParensWithin(|Pattern|) &ge; 2<sup>32</sup> - 1.
           </li>
           <li>
-            It is a Syntax Error if |Pattern| contains two or more |GroupSpecifier|s for which CapturingGroupName of |GroupSpecifier| is the same.
+            It is a Syntax Error if |Pattern| contains two distinct |GroupSpecifier|s _x_ and _y_ for which CapturingGroupName(_x_) is the same as CapturingGroupName(_y_) and such that CanBothParticipate(_x_, _y_) is *true*.
           </li>
         </ul>
         <emu-grammar>QuantifierPrefix :: `{` DecimalDigits `,` DecimalDigits `}`</emu-grammar>
@@ -34758,6 +34758,22 @@ <h1>
         </emu-alg>
       </emu-clause>
 
+      <emu-clause id="sec-patterns-static-semantics-can-both-participate" type="abstract operation">
+        <h1>
+          Static Semantics: CanBothParticipate (
+            _x_: a Parse Node,
+            _y_: a Parse Node,
+          ): a Boolean
+        </h1>
+        <dl class="header">
+        </dl>
+        <emu-alg>
+          1. Assert: _x_ and _y_ have the same enclosing |Pattern|.
+          1. If the enclosing |Pattern| contains a <emu-grammar>Disjunction :: Alternative `|` Disjunction</emu-grammar> Parse Node such that either _x_ is contained within the |Alternative| and _y_ is contained within the derived |Disjunction|, or _x_ is contained within the derived |Disjunction| and _y_ is contained within the |Alternative|, return *false*.
+          1. Return *true*.
+        </emu-alg>
+      </emu-clause>
+
       <emu-clause id="sec-patterns-static-semantics-capturing-group-number" type="sdo">
         <h1>Static Semantics: CapturingGroupNumber ( ): a positive integer</h1>
         <dl class="header">
@@ -35023,7 +35039,7 @@ <h1>
           1. Let _pattern_ be the |Pattern| containing _thisGroupName_.
           1. Let _result_ be a new empty List.
           1. For each |GroupSpecifier| _gs_ that _pattern_ contains, do
-            1. If the CapturingGroupName of _gs_ is the same value as _name_, then
+            1. If the CapturingGroupName of _gs_ is the same value as _name_, and CanBothParticipate(_thisGroupName_, _gs_) is *true*, then
               1. Append _gs_ to _result_.
           1. Return _result_.
         </emu-alg>
@@ -35598,7 +35614,7 @@ <h1>
         <emu-alg>
           1. Let _n_ be the CapturingGroupNumber of |DecimalEscape|.
           1. Assert: _n_ &le; _NcapturingParens_.
-          1. Return BackreferenceMatcher(_n_, _direction_).
+          1. Return BackreferenceMatcher(&laquo; _n_ &raquo;, _direction_).
         </emu-alg>
         <emu-note>
           <p>An escape sequence of the form `\\` followed by a non-zero decimal number _n_ matches the result of the _n_<sup>th</sup> set of capturing parentheses (<emu-xref href="#sec-notation"></emu-xref>). It is an error if the regular expression has fewer than _n_ capturing parentheses. If the regular expression has _n_ or more capturing parentheses but the _n_<sup>th</sup> one is *undefined* because it has not captured anything, then the backreference always succeeds.</p>
@@ -35618,10 +35634,11 @@ <h1>
         <emu-grammar>AtomEscape :: `k` GroupName</emu-grammar>
         <emu-alg>
           1. Let _matchingGroupSpecifiers_ be GroupSpecifiersThatMatch(|GroupName|).
-          1. Assert: _matchingGroupSpecifiers_ contains a single |GroupSpecifier|.
-          1. Let _groupSpecifier_ be the sole element of _matchingGroupSpecifiers_.
-          1. Let _parenIndex_ be CountLeftCapturingParensBefore(_groupSpecifier_).
-          1. Return BackreferenceMatcher(_parenIndex_, _direction_).
+          1. Let _parenIndices_ be a new empty List.
+          1. For each |GroupSpecifier| _groupSpecifier_ of _matchingGroupSpecifiers_, do
+            1. Let _parenIndex_ be CountLeftCapturingParensBefore(_groupSpecifier_).
+            1. Append _parenIndex_ to _parenIndices_.
+          1. Return BackreferenceMatcher(_parenIndices_, _direction_).
         </emu-alg>
 
         <emu-clause id="sec-runtime-semantics-charactersetmatcher-abstract-operation" type="abstract operation">
@@ -35657,19 +35674,22 @@ <h1>
         <emu-clause id="sec-backreference-matcher" type="abstract operation">
           <h1>
             BackreferenceMatcher (
-              _n_: a positive integer,
+              _ns_: a List of positive integers,
               _direction_: ~forward~ or ~backward~,
             ): a Matcher
           </h1>
           <dl class="header">
           </dl>
           <emu-alg>
-            1. Assert: _n_ &ge; 1.
-            1. Return a new Matcher with parameters (_x_, _c_) that captures _n_ and _direction_ and performs the following steps when called:
+            1. Return a new Matcher with parameters (_x_, _c_) that captures _ns_ and _direction_ and performs the following steps when called:
               1. Assert: _x_ is a State.
               1. Assert: _c_ is a Continuation.
               1. Let _cap_ be _x_'s _captures_ List.
-              1. Let _r_ be _cap_[_n_].
+              1. Let _r_ be *undefined*.
+              1. For each integer _n_ of _ns_, do
+                1. If _cap_[_n_] is not *undefined*, then
+                  1. Assert: _r_ is *undefined*.
+                  1. Set _r_ to _cap_[_n_].
               1. If _r_ is *undefined*, return _c_(_x_).
               1. Let _e_ be _x_'s _endIndex_.
               1. Let _rs_ be _r_'s _startIndex_.
@@ -36262,8 +36282,16 @@ <h1>
               1. Perform ! CreateDataPropertyOrThrow(_A_, ! ToString(𝔽(_i_)), _capturedValue_).
               1. If the _i_<sup>th</sup> capture of _R_ was defined with a |GroupName|, then
                 1. Let _s_ be the CapturingGroupName of that |GroupName|.
-                1. Perform ! CreateDataPropertyOrThrow(_groups_, _s_, _capturedValue_).
-                1. Append _s_ to _groupNames_.
+                1. Let _isMatchedElsewhere_ be *false*.
+                1. For each integer _j_ such that _j_ &ne; _i_, _j_ &ge; 1, and _j_ &le; _n_, do
+                  1. If the _j_<sup>th</sup> capture of _R_ was defined with a |GroupName|, then
+                    1. Let _sj_ be the CapturingGroupName of that |GroupName|.
+                    1. If _sj_ equals _s_ and the _j_<sup>th</sup> capture of _R_ is not *undefined*, set _isMatchedElsewhere_ to *true*.
+                1. If _isMatchedElsewhere_ is *false*, then
+                  1. Perform ! CreateDataPropertyOrThrow(_groups_, _s_, _capturedValue_).
+                  1. Append _s_ to _groupNames_.
+                1. Else,
+                  1. Append *undefined* to _groupNames_.
               1. Else,
                 1. Append *undefined* to _groupNames_.
             1. If _hasIndices_ is *true*, then