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

[Fznamznon]

Summary

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 https://github.com/llvm/llvm-project/issues/61758

[Fznamznon]

Fix test

[aaron.ballman]

Thank you for this! LGTM. but can you also be sure to update this table: https://github.com/llvm/llvm-project/blob/main/clang/docs/LanguageExtensions.rst?plain=1#L1429

[Fznamznon]

In D154334#4468158, @aaron.ballman wrote:

Thank you for this! LGTM. but can you also be sure to update this table: https://github.com/llvm/llvm-project/blob/main/clang/docs/LanguageExtensions.rst?plain=1#L1429

Hmm, I have a feeling that the table is for features that have __cpp_* macros defined for them. The features I intended to fix are described by the same doc here - https://github.com/llvm/llvm-project/blob/29f4c398717184a019791ed52d1d0d69ed5dabb6/clang/docs/LanguageExtensions.rst?plain=1#L1071 .

[aaron.ballman]

In D154334#4468404, @Fznamznon wrote:

In D154334#4468158, @aaron.ballman wrote:

Thank you for this! LGTM. but can you also be sure to update this table: https://github.com/llvm/llvm-project/blob/main/clang/docs/LanguageExtensions.rst?plain=1#L1429

Hmm, I have a feeling that the table is for features that have __cpp_* macros defined for them. The features I intended to fix are described by the same doc here - https://github.com/llvm/llvm-project/blob/29f4c398717184a019791ed52d1d0d69ed5dabb6/clang/docs/LanguageExtensions.rst?plain=1#L1071 .

Oh shoot, you're right, these don't have the typical feature testing macros from http://eel.is/c++draft/tab:cpp.predefined.ft ! Sorry for the noise, this is fine as-is.

[shafik]

Thank you or the quick fix!

clang/docs/ReleaseNotes.rst

 - Fix missing destructor calls and therefore memory leaks in generated code
   when an immediate invocation appears as a part of an expression that produces
   temporaries.
   (`#60709 <https://github.com/llvm/llvm-project/issues/60709>`_).
 - Fixed a missed integer overflow warning with temporary values.
   (`#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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a bug where attribute annotations on type specifiers (enums, classes,
   structs, unions, and scoped enums) were not properly ignored, resulting in

clang/include/clang/Basic/Features.def

 EXTENSION(c_alignof, true)
 EXTENSION(c_atomic, true)
 EXTENSION(c_generic_selections, true)
 EXTENSION(c_generic_selection_with_controlling_type, true)
 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)
 EXTENSION(cxx_local_type_template_args, LangOpts.CPlusPlus)
 EXTENSION(cxx_nonstatic_member_init, LangOpts.CPlusPlus)
 EXTENSION(cxx_override_control, LangOpts.CPlusPlus)
 EXTENSION(cxx_range_for, LangOpts.CPlusPlus)
 EXTENSION(cxx_reference_qualified_functions, LangOpts.CPlusPlus)

clang/test/Lexer/has_extension_cxx.cpp

 // RUN: %clang_cc1 -std=c++98 -E %s -o - | FileCheck %s
 // RUN: %clang_cc1 -std=c++11 -E %s -o - | FileCheck %s --check-prefix=CHECK11
 
 // CHECK: c_static_assert
 #if __has_extension(c_static_assert)
 int c_static_assert();
 #endif
 
 // CHECK: c_generic_selections
 #if __has_extension(c_generic_selections)
 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();
 #endif
 
 // CHECK: has_inline_namespaces
 #if __has_extension(cxx_inline_namespaces)
 int has_inline_namespaces();