Normative: allow duplicagte named capture groups

spec.html

@@ -34644,7 +34644,7 @@ <h1>Static Semantics: Early Errors</h1>
             It is a Syntax Error if _NcapturingParens_ &ge; 2<sup>32</sup> - 1.
           </li>
           <li>
-            It is a Syntax Error if |Pattern| contains multiple |GroupSpecifier|s whose enclosed |RegExpIdentifierName|s have the same CapturingGroupName.
+            It is a Syntax Error if |Pattern| contains two distinct |GroupSpecifier|s _x_ and _y_ whose enclosed |RegExpIdentifierName|s have the same CapturingGroupName and such that CanBothParticipate(_x_, _y_) is *true*.
           </li>
         </ul>
         <emu-grammar>QuantifierPrefix :: `{` DecimalDigits `,` DecimalDigits `}`</emu-grammar>
@@ -34656,7 +34656,7 @@ <h1>Static Semantics: Early Errors</h1>
         <emu-grammar>AtomEscape :: `k` GroupName</emu-grammar>
         <ul>
           <li>
-            It is a Syntax Error if the enclosing |Pattern| does not contain a |GroupSpecifier| with an enclosed |RegExpIdentifierName| whose CapturingGroupName equals the CapturingGroupName of the |RegExpIdentifierName| of this production's |GroupName|.
+            It is a Syntax Error if the enclosing |Pattern| does not contain a |GroupSpecifier| with an enclosed |RegExpIdentifierName| whose CapturingGroupName equals the CapturingGroupName of the |RegExpIdentifierName| of this production's |GroupName|, and such that CanBothParticipate(|AtomEscape|, |GroupSpecifier|) is *true*.
           </li>
         </ul>
         <emu-grammar>AtomEscape :: DecimalEscape</emu-grammar>
@@ -34724,6 +34724,22 @@ <h1>Static Semantics: Early Errors</h1>
         </ul>
       </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">
@@ -35566,7 +35582,7 @@ <h1>
         </emu-alg>
         <emu-grammar>AtomEscape :: `k` GroupName</emu-grammar>
         <emu-alg>
-          1. Search the enclosing |Pattern| for an instance of a |GroupSpecifier| containing a |RegExpIdentifierName| which has a CapturingGroupName equal to the CapturingGroupName of the |RegExpIdentifierName| contained in |GroupName|.
+          1. Search the enclosing |Pattern| for an instance of a |GroupSpecifier| containing a |RegExpIdentifierName| which has a CapturingGroupName equal to the CapturingGroupName of the |RegExpIdentifierName| contained in |GroupName|, and such that CanBothParticipate(|AtomEscape|, |GroupSpecifier|) is *true*.
           1. Assert: A unique such |GroupSpecifier| is found.
           1. Let _parenIndex_ be the number of left-capturing parentheses in the entire regular expression that occur to the left of the located |GroupSpecifier|. This is the total number of <emu-grammar>Atom :: `(` GroupSpecifier Disjunction `)`</emu-grammar> Parse Nodes prior to or enclosing the located |GroupSpecifier|, including its immediately enclosing |Atom|.
           1. Return BackreferenceMatcher(_parenIndex_, _direction_).
@@ -36210,7 +36226,13 @@ <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 the corresponding |RegExpIdentifierName|.
-                1. Perform ! CreateDataPropertyOrThrow(_groups_, _s_, _capturedValue_).
+                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 the corresponding |RegExpIdentifierName|.
+                    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_.