Updated Features.def to include c_fixed_enum. Updated enum.c test to check for support of enums with a fixed underlying type in C23 and <C23.

Author: Aidan

clang/include/clang/Basic/Features.def

@@ -131,6 +131,7 @@ FEATURE(objc_arc_fields, true)
 FEATURE(objc_arc_weak, LangOpts.ObjCWeak)
 FEATURE(objc_default_synthesize_properties, LangOpts.ObjC)
 FEATURE(objc_fixed_enum, LangOpts.ObjC)
+FEATURE(c_fixed_enum, true)
 FEATURE(objc_instancetype, LangOpts.ObjC)
 FEATURE(objc_kindof, LangOpts.ObjC)
 FEATURE(objc_modules, LangOpts.ObjC && LangOpts.Modules)

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
+    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 {
@@ -174,6 +185,7 @@ enum IncOverflow {
   V3 // pre-c23-warning {{incremented enumerator value which exceeds the range of 'int' is a C23 extension}}
 };
 
+
 #if __STDC_VERSION__ >= 202311L
 // FIXME: GCC picks __uint128_t as the underlying type for the enumeration
 // value and Clang picks unsigned long long.