[clang] [Clang] Implement C++26’s P2893R3 ‘Variadic friends’ (PR #101448)

[via cfe-commits]

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@@ -754,6 +754,10 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_multidimensional_subscript", "202211L");
     Builder.defineMacro("__cpp_auto_cast", "202110L");
   }
+  // C++26 features.
+  if (LangOpts.CPlusPlus26) {
+    Builder.defineMacro("__cpp_variadic_friend", "202403L");
+  }
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Sirraide wrote:

Update: `__cpp_pack_indexing` has since been added in a separate pr, but since we’re splitting this into multiple patches now, we won’t be adding a feature test macro for this just yet.

https://github.com/llvm/llvm-project/pull/101448