[clang] [clang-cl] Add support for [[msvc::constexpr]] C++11 attribute (PR #71300)

[Erich Keane via cfe-commits]

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@@ -2884,6 +2884,12 @@ def warn_cxx11_compat_constexpr_body_multiple_return : Warning<
   InGroup<CXXPre14Compat>, DefaultIgnore;
 def note_constexpr_body_previous_return : Note<
   "previous return statement is here">;
+def err_ms_constexpr_not_distinct : Error<
+  "[[msvc::constexpr]] cannot be applied to a %select{constexpr|consteval}0 function %1">;
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erichkeane wrote:

I don't think we add the `[[` and `]]` to any other diagnostic?  So I think we should jsust say: `attribute 'msvc::constexpr'` everywhere here.

https://github.com/llvm/llvm-project/pull/71300