[Clang] Set __cpp_explicit_this_parameter

There are not a lot of outstanding known issues
with deducing this (besides llvm#95112), so it
seems reasonable to claim full support.

Fixes llvm#82780

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -365,6 +365,7 @@ Bug Fixes to C++ Support
 - Fixed a bug in the substitution of empty pack indexing types. (#GH105903)
 - Clang no longer tries to capture non-odr used default arguments of template parameters of generic lambdas (#GH107048)
 - Fixed a bug where defaulted comparison operators would remove ``const`` from base classes. (#GH102588)
+- ``__cpp_explicit_this_parameter`` is now defined. (#GH82780)
 
 
 Bug Fixes to AST Handling

clang/lib/Frontend/InitPreprocessor.cpp

@@ -753,6 +753,7 @@ static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
     Builder.defineMacro("__cpp_if_consteval", "202106L");
     Builder.defineMacro("__cpp_multidimensional_subscript", "202211L");
     Builder.defineMacro("__cpp_auto_cast", "202110L");
+    Builder.defineMacro("__cpp_explicit_this_parameter", "202110L");
   }
 
   // We provide those C++23 features as extensions in earlier language modes, so

clang/test/Lexer/cxx-features.cpp

@@ -81,7 +81,7 @@
 #error "wrong value for __cpp_named_character_escapes"
 #endif
 
-#if check(explicit_this_parameter, 0, 0, 0, 0, 0, 0, 0)
+#if check(explicit_this_parameter, 0, 0, 0, 0, 0, 202110L, 202110L)
 #error "wrong value for __cpp_explicit_this_parameter"
 #endif