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

clang/docs/LanguageExtensions.rst

@@ -1986,7 +1986,7 @@ 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++
 
@@ -1996,7 +1996,13 @@ This specifies that the underlying type, which is used to store the enumeration
 value, is ``unsigned char``.
 
 Use ``__has_feature(objc_fixed_enum)`` to determine whether support for fixed
-underlying types is available in Objective-C.
+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
+
+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 {