[clang] [Clang] Add -Wimplicit-fallthrough to -Wextra for GCC compatibility (PR #149474)

[via cfe-commits]

https://github.com/lb90 created https://github.com/llvm/llvm-project/pull/149474

See also https://github.com/llvm/llvm-project/issues/58375

>From 42a89816f517718d517bcbd5f9d1bd5f629ac4bb Mon Sep 17 00:00:00 2001
From: Luca Bacci <luca.bacci982 at gmail.com>
Date: Fri, 18 Jul 2025 10:22:22 +0200
Subject: [PATCH] [Clang] Add -Wimplicit-fallthrough to -Wextra for GCC
 compatibility

---
 clang/docs/ReleaseNotes.rst                   | 2 ++
 clang/include/clang/Basic/DiagnosticGroups.td | 1 +
 2 files changed, 3 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ea16029268dba..f9b7240919785 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -716,6 +716,8 @@ Improvements to Clang's diagnostics
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
 
+- ``-Wimplicit-fallthrough`` was added to ``-Wextra`` for GCC compatibility.
+
 Improvements to Clang's time-trace
 ----------------------------------
 
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index ccb18aa37447e..6283c1ead791a 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1231,6 +1231,7 @@ def Extra : DiagGroup<"extra", [
     CastFunctionTypeMismatch,
     InitStringTooLongMissingNonString,
     WarnUnnecessaryVirtualSpecifier,
+    ImplicitFallthrough,
   ]>;
 
 def Most : DiagGroup<"most", [