[clang] [RFC] Initial implementation of P2719 (PR #113510)

[Oliver Hunt via cfe-commits]

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@@ -17389,6 +17389,19 @@ Sema::ActOnTag(Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
     Previous.clear();
   }
 
+  // I think DC check should be DC->isStdNamespace()?
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ojhunt wrote:

@erichkeane so I think this was a case where the test can fail incorrectly (albeit not in practice) and I was wanting confirmation of the fix, though I think the change should not be part of this PR :D

https://github.com/llvm/llvm-project/pull/113510