[diag] Silence -Wfixed-enum-extension in C23

[smeenai]

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

[llvmbot]

@llvm/pr-subscribers-clang

Changes

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

---

Full diff: https://github.com/llvm/llvm-project/pull/68060.diff

2 Files Affected:

• (modified) clang/lib/Parse/ParseDecl.cpp (+1-1)
• (modified) clang/test/Sema/fixed-enum.c (+8-4)

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index a1491596ef6145c..735da4aafae1ca5 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -5019,7 +5019,7 @@ void Parser::ParseEnumSpecifier(SourceLocation StartLoc, DeclSpec &DS,
         else if (getLangOpts().MicrosoftExt)
           Diag(ColonLoc, diag::ext_ms_c_enum_fixed_underlying_type)
               << BaseRange;
-        else
+        else if (!getLangOpts().C23)
           Diag(ColonLoc, diag::ext_clang_c_enum_fixed_underlying_type)
               << BaseRange;
       }
diff --git a/clang/test/Sema/fixed-enum.c b/clang/test/Sema/fixed-enum.c
index c77f5b0cbe79c5b..73a3fd2b09114c0 100644
--- a/clang/test/Sema/fixed-enum.c
+++ b/clang/test/Sema/fixed-enum.c
@@ -4,13 +4,17 @@
 // 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
 
 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 +25,19 @@ 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')}}

[erichkeane]

I believe this needs a release note as well (see docs/ReleaseNotes.rst), else this LGTM.