[diag] Silence -Wfixed-enum-extension in C23 (#68060)

The C23 standard supports enums with fixed underlying types (N3030 [1]),
so we shouldn't emit `-Wfixed-enum-extension` in C23 mode (since it's no
longer a Clang extension at that point).

[1]
https://thephd.dev/_vendor/future_cxx/papers/C%20-%20Enhanced%20Enumerations.html

Author: smeenai

clang/docs/ReleaseNotes.rst

@@ -219,6 +219,9 @@ Improvements to Clang's diagnostics
 - Clang now displays an improved diagnostic and a note when a defaulted special
   member is marked ``constexpr`` in a class with a virtual base class
   (`#64843: <https://github.com/llvm/llvm-project/issues/64843>`_).
+- ``-Wfixed-enum-extension`` and ``-Wmicrosoft-fixed-enum`` diagnostics are no longer
+  emitted when building as C23, since C23 standardizes support for enums with a
+  fixed underlying type.
 
 Bug Fixes in This Version
 -------------------------

clang/lib/Parse/ParseDecl.cpp

@@ -5009,7 +5009,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
 
       BaseRange = SourceRange(ColonLoc, DeclaratorInfo.getSourceRange().getEnd());
 
-      if (!getLangOpts().ObjC) {
+      if (!getLangOpts().ObjC && !getLangOpts().C23) {
         if (getLangOpts().CPlusPlus11)
           Diag(ColonLoc, diag::warn_cxx98_compat_enum_fixed_underlying_type)
               << BaseRange;

clang/test/Sema/fixed-enum.c

@@ -4,13 +4,18 @@
 // RUN: %clang_cc1 -Weverything -std=c11 -xc -DC11 -verify %s
 // RUN: %clang_cc1 -pedantic    -std=c11 -xc -DC11 -verify %s
 // RUN: %clang_cc1 -Weverything -std=c11 -xc -fms-extensions -DMS -verify %s
+// RUN: %clang_cc1 -Weverything -std=c2x -xc -DC23 -verify %s
+// RUN: %clang_cc1 -pedantic    -std=c2x -xc -DC23 -verify %s
+// RUN: %clang_cc1 -Weverything -std=c23 -xc -DC23 -verify %s
+// RUN: %clang_cc1 -pedantic    -std=c23 -xc -DC23 -verify %s
+// RUN: %clang_cc1 -Weverything -std=c23 -xc -fms-extensions -DC23 -verify %s
 
 enum X : int {e};
 #if defined(CXX11)
 // expected-warning@-2{{enumeration types with a fixed underlying type are incompatible with C++98}}
 #elif defined(CXX03)
 // expected-warning@-4{{enumeration types with a fixed underlying type are a C++11 extension}}
-#elif defined(OBJC)
+#elif defined(OBJC) || defined(C23)
 // No diagnostic
 #elif defined(C11)
 // expected-warning@-8{{enumeration types with a fixed underlying type are a Clang extension}}
@@ -21,19 +26,19 @@ enum X : int {e};
 // Don't warn about the forward declaration in any language mode.
 enum Fwd : int;
 enum Fwd : int { e2 };
-#ifndef OBJC
+#if !defined(OBJC) && !defined(C23)
 // expected-warning@-3 {{enumeration types with a fixed underlying type}}
 // expected-warning@-3 {{enumeration types with a fixed underlying type}}
 #endif
 
 // Always error on the incompatible redeclaration.
 enum BadFwd : int;
-#ifndef OBJC
+#if !defined(OBJC) && !defined(C23)
 // expected-warning@-2 {{enumeration types with a fixed underlying type}}
 #endif
 // expected-note@-4 {{previous declaration is here}}
 enum BadFwd : char { e3 };
-#ifndef OBJC
+#if !defined(OBJC) && !defined(C23)
 // expected-warning@-2 {{enumeration types with a fixed underlying type}}
 #endif
 // expected-error@-4 {{enumeration redeclared with different underlying type 'char' (was 'int')}}