[clang] [Clang] disallow # operators in attribute argument lists (PR #147308)

[Erich Keane via cfe-commits]

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@@ -488,6 +488,13 @@ unsigned Parser::ParseAttributeArgsCommon(
   bool AttributeHasVariadicIdentifierArg =
       attributeHasVariadicIdentifierArg(*AttrName, Form.getSyntax(), ScopeName);
 
+  if (Tok.is(tok::hash) || Tok.is(tok::hashhash)) {
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erichkeane wrote:

Based on my reading of the C standard, we should ONLY disallow this in C++ mode.

https://github.com/llvm/llvm-project/pull/147308