[clang] Add __has_extension () for C++11 features

Add `__has_extension (cxx_defaulted_functions)` and
`__has_extension (cxx_default_function_template_args)` since they are
accepted in C++98 mode as extensions.

Fixes #61758

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D154334

clang/docs/ReleaseNotes.rst

@@ -565,6 +565,9 @@ Bug Fixes in This Version
   (`#63629 <https://github.com/llvm/llvm-project/issues/63629>`_)
 - Fixed parsing of elaborated type specifier inside of a new expression.
   (`#34341 <https://github.com/llvm/llvm-project/issues/34341>`_)
+- Clang now correctly evaluates ``__has_extension (cxx_defaulted_functions)``
+  and ``__has_extension (cxx_default_function_template_args)`` to 1.
+  (`#61758 <https://github.com/llvm/llvm-project/issues/61758>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Basic/Features.def

@@ -247,6 +247,8 @@ EXTENSION(c_static_assert, true)
 EXTENSION(c_thread_local, PP.getTargetInfo().isTLSSupported())
 // C++11 features supported by other languages as extensions.
 EXTENSION(cxx_atomic, LangOpts.CPlusPlus)
+EXTENSION(cxx_default_function_template_args, LangOpts.CPlusPlus)
+EXTENSION(cxx_defaulted_functions, LangOpts.CPlusPlus)
 EXTENSION(cxx_deleted_functions, LangOpts.CPlusPlus)
 EXTENSION(cxx_explicit_conversions, LangOpts.CPlusPlus)
 EXTENSION(cxx_inline_namespaces, LangOpts.CPlusPlus)

clang/test/Lexer/has_extension_cxx.cpp

@@ -11,6 +11,16 @@ int c_static_assert();
 int c_generic_selections();
 #endif
 
+// CHECK: has_default_function_template_args
+#if __has_extension(cxx_default_function_template_args)
+int has_default_function_template_args();
+#endif
+
+// CHECK: has_defaulted_functions
+#if __has_extension(cxx_defaulted_functions)
+int has_defaulted_functions();
+#endif
+
 // CHECK: has_deleted_functions
 #if __has_extension(cxx_deleted_functions)
 int has_deleted_functions();