[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++
 
@@ -1998,6 +1998,9 @@ 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.
+
 Interoperability with C++11 lambdas
 -----------------------------------
 

clang/include/clang/Basic/Features.def

@@ -308,6 +308,10 @@ EXTENSION(datasizeof, LangOpts.CPlusPlus)
 
 FEATURE(cxx_abi_relative_vtable, LangOpts.CPlusPlus && LangOpts.RelativeCXXABIVTables)
 
+//Fixed enum feature and extension, to be relocated in this file
+FEATURE(c_fixed_enum, true)    
+EXTENSION(c_fixed_enum, true)  
+
 // CUDA/HIP Features
 FEATURE(cuda_noinline_keyword, LangOpts.CUDA)
 EXTENSION(cuda_implicit_host_device_templates, LangOpts.CUDA && LangOpts.OffloadImplicitHostDeviceTemplates)

clang/test/Sema/enum.c

@@ -121,6 +121,15 @@ 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
+    typedef enum : unsigned char { Pink, Black, Cyan } Color;
+#else
+    _Static_assert(__has_extension(c_fixed_enum), "Ensure language extension support for enumerations with a fixed underlying type in <C23");
+    typedef enum : unsigned char { Pink, Black, Cyan } Color; // expected-warning {{enumeration types with a fixed underlying type are a C23 extension}}
+#endif
+
 // PR28903
 // In C it is valid to define tags inside enums.
 struct PR28903 {