[clang] [Clang][C++26] Implement Pack Indexing (P2662R3). (PR #72644)

[Erich Keane via cfe-commits]

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@@ -978,6 +978,20 @@ bool Parser::ConsumeAndStoreFunctionPrologue(CachedTokens &Toks) {
       } else {
         break;
       }
+      // Pack indexing
+      if (getLangOpts().CPlusPlus26 && Tok.is(tok::ellipsis) &&
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erichkeane wrote:

While I think allowing this in older modes as an extension is perhaps a bridge-too-far, we should probably diagnose the 'else' case here (that is, Tok is ellipsis, NextTok is l_square) as 'not allowed in pre-c++26', and see if we can recover by consuming the tokens anyway.

https://github.com/llvm/llvm-project/pull/72644