[Clang] define __cpp_named_character_escapes

[royjacobson]

Summary

Define the feature test macro for named character escapes.
I assume this was not done because it was implemented before formally accepted, right? cxx_status says the paper is implemented.

[royjacobson]

rebase

[cor3ntin]

Thanks for catching that, I missed there was a feature test macro for it.
It doesn't really make sense for it to be a feature test macro (given WG21's current policies) so I'll make raise an issue about that but I'm happy to approve the change in the meantime

[royjacobson]

Remove literal suffix in accordance with the rest of the test

[royjacobson]

In D134898#3825255, @cor3ntin wrote:

Thanks for catching that, I missed there was a feature test macro for it.
It doesn't really make sense for it to be a feature test macro (given WG21's current policies) so I'll make raise an issue about that but I'm happy to approve the change in the meantime

Then I hope we won't have to revert this in two years or something :)

But in the meantime I agree this is what users will expect so it's better to define it than not to.

clang/lib/Frontend/InitPreprocessor.cpp

static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   // C++2b features.
   if (LangOpts.CPlusPlus2b) {
     Builder.defineMacro("__cpp_implicit_move", "202011L");
     Builder.defineMacro("__cpp_size_t_suffix", "202011L");
     Builder.defineMacro("__cpp_if_consteval", "202106L");
     Builder.defineMacro("__cpp_multidimensional_subscript", "202110L");
   }
 
-  // We provide this as an extension in earlier language modes, so we
-  // also define the macro.
+  // We provide those C++2b features as extensions in earlier language modes, so
+  // we also define their feature test macros.
   if (LangOpts.CPlusPlus11)
     Builder.defineMacro("__cpp_static_call_operator", "202207L");
+  Builder.defineMacro("__cpp_named_character_escapes", "202207L");
 
   if (LangOpts.Char8)
     Builder.defineMacro("__cpp_char8_t", "201811L");
   Builder.defineMacro("__cpp_impl_destroying_delete", "201806L");
 
   // TS features.
   if (LangOpts.Coroutines)
     Builder.defineMacro("__cpp_coroutines", "201703L");

clang/test/Lexer/cxx-features.cpp

 #if check(multidimensional_subscript, 0, 0, 0, 0, 0, 202110)
 #error "wrong value for __cpp_multidimensional_subscript"
 #endif
 
 #if check(static_call_operator, 0, 202207, 202207, 202207, 202207, 202207)
 #error "wrong value for __cpp_static_call_operator"
 #endif
 
-#if check(named_character_escapes, 0, 0, 0, 0, 0, 0)
+#if check(named_character_escapes, 202207L, 202207L, 202207L, 202207L, 202207L, 202207L)
 #error "wrong value for __cpp_named_character_escapes"
 #endif
 
 #if check(explicit_this_parameter, 0, 0, 0, 0, 0, 0)
 #error "wrong value for __cpp_explicit_this_parameter"
 #endif
 
 // --- C++20 features ---