[clang] [Clang] Warn with -Wpre-c23-compat instead of -Wpre-c++17-compat for u8 character literals in C23 (PR #97210)

[Mital Ashok via cfe-commits]

https://github.com/MitalAshok created https://github.com/llvm/llvm-project/pull/97210

None

>From c6ee783243e1888074778e2cb6de05df41cc8333 Mon Sep 17 00:00:00 2001
From: Mital Ashok <mital at mitalashok.co.uk>
Date: Sun, 30 Jun 2024 12:55:04 +0100
Subject: [PATCH] [Clang] Warn with -Wpre-c23-compat instead of
 -Wpre-c++17-compat for u8 character literals in C23

---
 clang/docs/ReleaseNotes.rst                     | 2 ++
 clang/include/clang/Basic/DiagnosticLexKinds.td | 3 +++
 clang/lib/Lex/Lexer.cpp                         | 4 +++-
 clang/test/Sema/pre-c2x-compat.c                | 1 +
 4 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index c720e47dbe35b..d2939ef7a875c 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -627,6 +627,8 @@ Improvements to Clang's diagnostics
   used rather than when they are needed for constant evaluation or when code is generated for them.
   The check is now stricter to prevent crashes for some unsupported declarations (Fixes #GH95495).
 
+- Clang now warns for u8 character literals used in C23 with ``-Wpre-c23-compat`` instead of ``-Wpre-c++17-compat``.
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 12d7b8c0205ee..fc14bb6aa2165 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -283,6 +283,9 @@ def warn_cxx98_compat_unicode_literal : Warning<
 def warn_cxx14_compat_u8_character_literal : Warning<
   "unicode literals are incompatible with C++ standards before C++17">,
   InGroup<CXXPre17Compat>, DefaultIgnore;
+def warn_c17_compat_u8_character_literal : Warning<
+  "unicode literals are incompatible with C standards before C23">,
+  InGroup<CPre23Compat>, DefaultIgnore;
 def warn_cxx11_compat_user_defined_literal : Warning<
   "identifier after literal will be treated as a user-defined literal suffix "
   "in C++11">, InGroup<CXX11Compat>, DefaultIgnore;
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index e59c7805b3862..c61d81e93d990 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -2428,7 +2428,9 @@ bool Lexer::LexCharConstant(Token &Result, const char *CurPtr,
                           ? diag::warn_cxx98_compat_unicode_literal
                           : diag::warn_c99_compat_unicode_literal);
     else if (Kind == tok::utf8_char_constant)
-      Diag(BufferPtr, diag::warn_cxx14_compat_u8_character_literal);
+      Diag(BufferPtr, LangOpts.CPlusPlus
+                          ? diag::warn_cxx14_compat_u8_character_literal
+                          : diag::warn_c17_compat_u8_character_literal);
   }
 
   char C = getAndAdvanceChar(CurPtr, Result);
diff --git a/clang/test/Sema/pre-c2x-compat.c b/clang/test/Sema/pre-c2x-compat.c
index fad472f1f72d5..15bb9b58349fa 100644
--- a/clang/test/Sema/pre-c2x-compat.c
+++ b/clang/test/Sema/pre-c2x-compat.c
@@ -1,3 +1,4 @@
 // RUN: %clang_cc1 %s -std=c2x -Wpre-c2x-compat -pedantic -fsyntax-only -verify
 
 int digit_seps = 123'456; // expected-warning {{digit separators are incompatible with C standards before C23}}
+unsigned char u8_char = u8'x'; // expected-warning {{unicode literals are incompatible with C standards before C23}}