[clang][sema] Add support and documentation for __has_extension(c_fixed_enum) (#117507)

This PR addresses #116880 

Updated
[LanguageExtensions.rst](https://github.com/llvm/llvm-project/blob/main/clang/docs/LanguageExtensions.rst)
to include support for C++11 enumerations with a fixed underlying type
in C. Included a note that this is only a language extension prior to
C23.

Updated
[Features.def](https://github.com/llvm-mirror/clang/blob/master/include/clang/Basic/Features.def)
to support for `__has_extension(c_fixed_enum)` by added it as a feature
(for C23) and an extension (for <C23).

Updated
[enum.c](https://github.com/llvm/llvm-project/blob/main/clang/test/Sema/enum.c)
to ensure support of C++11 enumerations with a fixed underlying type in
both <C23 and C23, as well as the functionality of
`__has_extension(c_fixed_enum)`.

---

In enum.c, I encountered a warning when testing enumerations with a
fixed underlying type in pre-C23 modes. Specifically, the test produces
the warning: `enumeration types with a fixed underlying type are a C23
extension`. I am unsure if this warning is expected behavior, as
enumerations with a fixed underlying type should behave identically in
pre-C23 and C23 modes. I expected that adding `c_fixed_enum` as an
extension would fix this warning. Feedback on whether this is correct or
requires adjustment would be appreciated.

I was also unsure of the best location for the additions in
`Features.def`, I would appreciate advice on this as well

Note that this is my first PR to LLVM, so please liberally critique it!

Author: AidanGoldfarb

clang/docs/LanguageExtensions.rst

@@ -1989,7 +1989,7 @@ Enumerations with a fixed underlying type
 -----------------------------------------
 
 Clang provides support for C++11 enumerations with a fixed underlying type
-within Objective-C.  For example, one can write an enumeration type as:
+within Objective-C and C `prior to C23 <https://open-std.org/JTC1/SC22/WG14/www/docs/n3030.htm>`_.  For example, one can write an enumeration type as:
 
 .. code-block:: c++
 
@@ -2001,6 +2001,14 @@ value, is ``unsigned char``.
 Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
 underlying types is available in Objective-C.
 
+Use ``__has_extension(c_fixed_enum)`` to determine whether support for fixed
+underlying types is available in C prior to C23. This will also report ``true`` in C23
+and later modes as the functionality is available even if it's not an extension in
+those modes.
+
+Use ``__has_feature(c_fixed_enum)`` to determine whether support for fixed
+underlying types is available in C23 and later.
+
 Interoperability with C++11 lambdas
 -----------------------------------
 

clang/include/clang/Basic/Features.def

@@ -163,6 +163,8 @@ FEATURE(c_atomic, LangOpts.C11)
 FEATURE(c_generic_selections, LangOpts.C11)
 FEATURE(c_static_assert, LangOpts.C11)
 FEATURE(c_thread_local, LangOpts.C11 &&PP.getTargetInfo().isTLSSupported())
+// C23 features
+FEATURE(c_fixed_enum, LangOpts.C23)
 // C++11 features
 FEATURE(cxx_access_control_sfinae, LangOpts.CPlusPlus11)
 FEATURE(cxx_alias_templates, LangOpts.CPlusPlus11)
@@ -269,6 +271,7 @@ EXTENSION(c_static_assert, true)
 EXTENSION(c_thread_local, PP.getTargetInfo().isTLSSupported())
 // C23 features supported by other languages as extensions
 EXTENSION(c_attributes, true)
+EXTENSION(c_fixed_enum, true)
 // C++11 features supported by other languages as extensions.
 EXTENSION(cxx_atomic, LangOpts.CPlusPlus)
 EXTENSION(cxx_default_function_template_args, LangOpts.CPlusPlus)

clang/test/Sema/enum.c

@@ -121,6 +121,17 @@ int NegativeShortTest[NegativeShort == -1 ? 1 : -1];
 enum Color { Red, Green, Blue }; // expected-note{{previous use is here}}
 typedef struct Color NewColor; // expected-error {{use of 'Color' with tag type that does not match previous declaration}}
 
+// Enumerations with a fixed underlying type. 
+// https://github.com/llvm/llvm-project/issues/116880
+#if __STDC_VERSION__ >= 202311L
+  static_assert(__has_feature(c_fixed_enum));
+  static_assert(__has_extension(c_fixed_enum)); // Matches behavior for c_alignas, etc
+#else
+  _Static_assert(__has_extension(c_fixed_enum), "");
+  _Static_assert(!__has_feature(c_fixed_enum), "");
+#endif
+typedef enum : unsigned char { Pink, Black, Cyan } Color; // pre-c23-warning {{enumeration types with a fixed underlying type are a C23 extension}}
+
 // PR28903
 // In C it is valid to define tags inside enums.
 struct PR28903 {