[clang] Give the __COUNTER__ pedantic diagnostic a group (PR #218669)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman updated https://github.com/llvm/llvm-project/pull/218669

>From 0b20f92079d1bd732535cc694e27da57a9177170 Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Tue, 25 Aug 2026 08:16:28 -0400
Subject: [PATCH 1/2] Give the __COUNTER__ pedantic diagnostic a group

Adding the pedantic diagnostic after supporting the extension for so
long without correctly diagnosing it is disruptive, so this gives folks
a way to disable just the __COUNTER__ pedantic diagnostic without
losing all other pedantic diagnostics.

Fixes #196557
---
 clang/include/clang/Basic/DiagnosticGroups.td   | 3 ++-
 clang/include/clang/Basic/DiagnosticLexKinds.td | 3 ++-
 clang/test/C/C2y/n3457.c                        | 3 +++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index 9ee0b61a96a32..7af6a3b2d284c 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -1026,6 +1026,7 @@ def SuspiciousMemaccess : DiagGroup<"suspicious-memaccess",
    NonTrivialMemaccess, MemsetTransposedArgs, SuspiciousBzero]>;
 def StaticInInline : DiagGroup<"static-in-inline">;
 def StaticLocalInInline : DiagGroup<"static-local-in-inline">;
+def CounterExt : DiagGroup<"counter-extension">;
 def UniqueObjectDuplication : DiagGroup<"unique-object-duplication"> {
   code Documentation = [{
 Warns when objects which are supposed to be globally unique might get duplicated
@@ -1616,7 +1617,7 @@ def C23 : DiagGroup<"c23-extensions", [VariadicMacroArgumentsOmitted]>;
 def : DiagGroup<"c2x-extensions", [C23]>;
 
 // A warning group for warnings about using C2y features as extensions.
-def C2y : DiagGroup<"c2y-extensions", [StaticInInline]>;
+def C2y : DiagGroup<"c2y-extensions", [StaticInInline, CounterExt]>;
 
 // Previously supported warning group which is no longer pertinent as binary
 // literals are a C++14 and C23 extension now instead of a GNU extension.
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index bdb78ed80b7b4..291ebba95b9cf 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -93,7 +93,8 @@ def err_conflict_marker : Error<"version control conflict marker in file">;
 def err_counter_overflow : Error<
   "'__COUNTER__' value cannot exceed 2'147'483'647">;
 def ext_counter : Extension<
-  "'__COUNTER__' is a C2y extension">, InGroup<C2y>, SuppressInSystemMacro;
+  "'__COUNTER__' is a C2y extension">,
+  InGroup<CounterExt>, SuppressInSystemMacro;
 def warn_counter : Warning<
   "'__COUNTER__' is incompatible with standards before C2y">,
   InGroup<CPre2yCompat>, DefaultIgnore, SuppressInSystemMacro;
diff --git a/clang/test/C/C2y/n3457.c b/clang/test/C/C2y/n3457.c
index d71a3f37e1343..a4733e9fed8b8 100644
--- a/clang/test/C/C2y/n3457.c
+++ b/clang/test/C/C2y/n3457.c
@@ -1,6 +1,9 @@
 // RUN: %clang_cc1 -verify=ext -std=c23 -pedantic %s
 // RUN: %clang_cc1 -verify=ext -pedantic -x c++ %s
 // RUN: %clang_cc1 -verify=pre -std=c2y -pedantic -Wpre-c2y-compat %s
+// RUN: %clang_cc1 -verify=ok  -std=c23 -pedantic -Wno-counter-extension %s
+// RUN: %clang_cc1 -verify=ok  -std=c23 -Wno-counter-extension -pedantic-errors %s
+// ok-no-diagnostics
 
 /* WG14 N3457: Clang 22
  * The __COUNTER__ predefined macro

>From 9a2e67a96db5a4eb4b9894febd46b4c20cf4cfcf Mon Sep 17 00:00:00 2001
From: Aaron Ballman <aaron at aaronballman.com>
Date: Tue, 25 Aug 2026 09:06:02 -0400
Subject: [PATCH 2/2] Add release note

---
 clang/docs/ReleaseNotes.md | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 8cc8eb5f80066..a15c80dd5efbe 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -421,6 +421,11 @@ features cannot lower the translation-unit ABI level;
 - `-Wc++98-compat` now diagnoses explicit conversion functions in C++20 and
   later, matching the behavior in C++11 through C++17. (#GH161689)
 
+- Added `-Wcounter-extension` as a diagnostic group under `-Wc2y-extensions` to
+  control `__COUNTER__` being diagnosed as an extension. This allows `-pedantic`
+  users to disable the diagnostic with `-Wno-counter-extension` without having
+  to disable all pedantic diagnostics. (#GH196557)
+
 ### Improvements to Clang's time-trace
 
 ### Improvements to Coverage Mapping