[clang-tools-extra] [clang-tidy] Change IgnoreExternC default to true in modernize-use-using (PR #199189)

[via cfe-commits]

https://github.com/voyager-jhk updated https://github.com/llvm/llvm-project/pull/199189

>From 2a465c1ef7ab4ed081a23cbbb6831ac1eedbba8e Mon Sep 17 00:00:00 2001
From: voyager-jhk <voyager.lpq at gmail.com>
Date: Fri, 22 May 2026 16:57:44 +0800
Subject: [PATCH] [clang-tidy] Change IgnoreExternC default to true in
 modernize-use-using

This prevents generating invalid C code in mixed-language headers by leaving
`typedef` declarations inside `extern "C"` blocks intact by default.

Fixes #141394
---
 clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp    | 2 +-
 clang-tools-extra/docs/ReleaseNotes.rst                     | 6 ++++++
 .../docs/clang-tidy/checks/modernize/use-using.rst          | 2 +-
 .../checkers/modernize/use-using-ignore-extern-c.cpp        | 4 +++-
 .../test/clang-tidy/checkers/modernize/use-using.cpp        | 2 --
 5 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp b/clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp
index 327fe02735468..cbb3babe9d472 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp
+++ b/clang-tools-extra/clang-tidy/modernize/UseUsingCheck.cpp
@@ -235,7 +235,7 @@ static constexpr StringRef DeclStmtName = "decl-stmt";
 UseUsingCheck::UseUsingCheck(StringRef Name, ClangTidyContext *Context)
     : ClangTidyCheck(Name, Context),
       IgnoreMacros(Options.get("IgnoreMacros", true)),
-      IgnoreExternC(Options.get("IgnoreExternC", false)) {}
+      IgnoreExternC(Options.get("IgnoreExternC", true)) {}
 
 void UseUsingCheck::storeOptions(ClangTidyOptions::OptionMap &Opts) {
   Options.store(Opts, "IgnoreMacros", IgnoreMacros);
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
index e60800fd913d1..b3a2e89ac90c6 100644
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -49,6 +49,10 @@ Major New Features
 Potentially Breaking Changes
 ----------------------------
 
+- The :doc:`modernize-use-using <clang-tidy/checks/modernize/use-using>` check
+  now sets the `IgnoreExternC` option to `true` by default. The check will
+  no longer transform ``typedef``\ s within ``extern "C"`` blocks.
+
 - Deprecated the :program:`clang-tidy` check :doc:`performance-faster-string-find
   <clang-tidy/checks/performance/faster-string-find>`. It has been renamed to
   :doc:`performance-prefer-single-char-overloads
@@ -580,6 +584,8 @@ Changes in existing checks
 
   - Preserve inline comment blocks that appear between the ``typedef``'s parts.
 
+  - The `IgnoreExternC` option is now set to `true` by default.
+
 - Improved :doc:`performance-enum-size
   <clang-tidy/checks/performance/enum-size>` check:
 
diff --git a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-using.rst b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-using.rst
index 9eedf20cb9723..82704fbbfedb0 100644
--- a/clang-tools-extra/docs/clang-tidy/checks/modernize/use-using.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/modernize/use-using.rst
@@ -49,4 +49,4 @@ Options
 .. option:: IgnoreExternC
 
    If set to `true`, the check will not give warning inside ``extern "C"``
-   scope. Default is `false`
+   scope. Default is `true`.
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using-ignore-extern-c.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using-ignore-extern-c.cpp
index 6a845a0bcc350..ac7c767c610e3 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using-ignore-extern-c.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using-ignore-extern-c.cpp
@@ -1,9 +1,11 @@
-// RUN: %check_clang_tidy %s modernize-use-using %t -- -config="{CheckOptions: {modernize-use-using.IgnoreExternC: true}}" -- -I %S/Input/use-using/
+// RUN: %check_clang_tidy %s modernize-use-using %t -- -config="{CheckOptions: {modernize-use-using.IgnoreExternC: false}}" -- -I %S/Input/use-using/
 
 // Some Header
 extern "C" {
 
 typedef int NewInt;
+// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use 'using' instead of 'typedef' [modernize-use-using]
+// CHECK-FIXES: using NewInt = int;
 }
 
 extern "C++" {
diff --git a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using.cpp b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using.cpp
index 232dd33f35f37..36dc93faca461 100644
--- a/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/modernize/use-using.cpp
@@ -330,8 +330,6 @@ typedef class ISSUE_67529_1 *ISSUE_67529;
 extern "C" {
 
 typedef int InExternC;
-// CHECK-MESSAGES: :[[@LINE-1]]:1: warning: use 'using' instead of 'typedef' [modernize-use-using]
-// CHECK-FIXES: using InExternC = int;
 
 }