[clang] [Clang] restrict use of attribute names reserved by the C++ standard (PR #106036)

[Jonathan Wakely via cfe-commits]

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@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx11 -pedantic -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx14 -pedantic -std=c++14 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx17 -pedantic -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx20 -pedantic -std=c++20 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx23 -pedantic -std=c++23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=expected,cxx26 -pedantic -std=c++26 %s
+
+#define noreturn 1           // cxx11-warning {{keyword is hidden by macro definition}}
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jwakely wrote:

> How would that work? `int noreturn = 1;` is valid code AFAICT.

Yes, for attributes, that's true. For keywords it's not though.

The rule was changed between C++03 and C++11 to include #undef, I don't know why though.

I still don't see any advantage in allowing users to `#undef noreturn`.

https://github.com/llvm/llvm-project/pull/106036