[clang] [Clang][Parse] Fix assertion when annotating a failed decltype-specifier (PR #211221)

[Akash Agrawal via cfe-commits]

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@@ -1136,6 +1136,20 @@ void Parser::AnnotateExistingDecltypeSpecifier(const DeclSpec &DS,
   // make sure we have a token we can turn into an annotation token
   if (PP.isBacktrackEnabled()) {
     PP.RevertCachedTokens(1);
+    if (DS.getTypeSpecType() == TST_error && PP.hasCachedTokenLocation()) {
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akashagrwl wrote:

Reworked it — the fix now drops the invalid decltype-specifier right in ParseOptionalCXXScopeSpecifier, with an early return false on TST_error immediately after ParseDecltypeSpecifier, before any annotation happens. The hasCachedTokenLocation() helper and the AnnotateExistingDecltypeSpecifier special-casing are both removed — there's no longer a code path that annotates an errored DeclSpec, so the stale-EndLoc problem this was patching around can't occur. This mirrors the adjacent pack-indexing branch, which already bails out the same way on TST_error. Also added a release note.

https://github.com/llvm/llvm-project/pull/211221