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

[Erich Keane via cfe-commits]

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@@ -12,5 +12,5 @@
 [[using clang:]] extern int n; // ok
 [[using blah: clang::optnone]] extern int n; // expected-error {{attribute with scope specifier cannot follow}} expected-warning {{only applies to functions}}
 
-[[using clang: unknown_attr]] extern int n; // expected-warning {{unknown attribute}}
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erichkeane wrote:

This shows the exact problem here that I have above.  We KNOW 'clang' (in fact, we OWN it).  We should diagnose this as an unknown attribute, not as an unknown namespace.

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