[clang] [clang]: implement std::meta::is_type (PR #211653)

[Ambrose Leeb via cfe-commits]

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@@ -766,6 +766,10 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   Builder.defineMacro("__cpp_variadic_friend", "202403L");
   Builder.defineMacro("__cpp_trivial_relocatability", "202502L");
 
+  // C++26 reflection.
+  if (LangOpts.Reflection)
+    Builder.defineMacro("__cpp_impl_reflection", "202506L");
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Sirraide wrote:

I don’t believe we should define this until reflexion is _fully_ implemented and tested. This is what we’ve been doing for other language features and their feature-test macros

https://github.com/llvm/llvm-project/pull/211653