[clang] [Clang] raise extension warning for unknown namespaced attributes (PR #120925)

[Erich Keane via cfe-commits]

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@@ -179,6 +179,8 @@ def err_opencl_unknown_type_specifier : Error<
 
 def warn_unknown_attribute_ignored : Warning<
   "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
+def ext_unknown_attribute_ignored : Extension<
+  "unknown attribute %0 ignored">, InGroup<UnknownAttributes>;
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erichkeane wrote:

I would expect this to be a normal warning.  The difference between `clang::Unknown` and `Unknown::UnknownAttr` are the same severity/warning type/etc.  A different MESSAGE is sensible, and a different warning GROUP is equally as sensible, but one being a warning and the other an extension seems odd.

https://github.com/llvm/llvm-project/pull/120925