From 92bd7664d64a45324900a5935d6e0af7ca60b922 Mon Sep 17 00:00:00 2001
From: esadomer <54475303+esadomer@users.noreply.github.com>
Date: Tue, 13 Oct 2026 19:50:34 +0300
Subject: [PATCH 1/2] gh-159338: Default the warning checker's optional path
 prefix to an empty string

---
 Lib/test/test_tools/test_check_warnings.py | 30 ++++++++++++++++++++++
 Tools/build/check_warnings.py              |  1 +
 2 files changed, 31 insertions(+)
 create mode 100644 Lib/test/test_tools/test_check_warnings.py

diff --git a/Lib/test/test_tools/test_check_warnings.py b/Lib/test/test_tools/test_check_warnings.py
new file mode 100644
index 000000000000000..e8745b421784062
--- /dev/null
+++ b/Lib/test/test_tools/test_check_warnings.py
@@ -0,0 +1,30 @@
+import importlib
+import unittest
+from pathlib import Path
+from test.support import captured_stdout, os_helper
+
+from test.test_tools import imports_under_tool, skip_if_missing
+
+
+skip_if_missing("build")
+with imports_under_tool("build"):
+    check_warnings = importlib.import_module("check_warnings")
+
+
+class CheckWarningsTests(unittest.TestCase):
+    def test_cli_without_path_prefix(self):
+        with os_helper.temp_dir() as directory:
+            output = Path(directory, "compiler.txt")
+            output.write_text("source.c:1:2: warning: example [-Wunused]\n",
+                              encoding="utf-8")
+            ignore = Path(directory, "ignore.txt")
+            ignore.write_text("source.c 1\n", encoding="utf-8")
+            args = ["-c", str(output), "-i", str(ignore), "-t", "gcc"]
+            with captured_stdout():
+                self.assertEqual(check_warnings.main(args), 0)
+                self.assertEqual(check_warnings.main(
+                    args + ["--fail-on-regression"]), 0)
+
+
+if __name__ == "__main__":
+    unittest.main()
diff --git a/Tools/build/check_warnings.py b/Tools/build/check_warnings.py
index 44ccf9708ad72f0..c568bdf82ed3003 100644
--- a/Tools/build/check_warnings.py
+++ b/Tools/build/check_warnings.py
@@ -267,6 +267,7 @@ def main(argv: list[str] | None = None) -> int:
         "-p",
         "--path-prefix",
         type=str,
+        default="",
         help="Path prefix to remove from the start of file paths"
         " in compiler output",
     )

From ee32a4871ed99d4723e9148a66f7f862c635dbbd Mon Sep 17 00:00:00 2001
From: esadomer <54475303+esadomer@users.noreply.github.com>
Date: Tue, 13 Oct 2026 20:09:00 +0300
Subject: [PATCH 2/2] gh-159338: Add NEWS entry for
 warnings-default-path-prefix

---
 .../Tools-Demos/2026-10-13-20-08-59.gh-issue-159338.wPthPr.rst  | 2 ++
 1 file changed, 2 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Tools-Demos/2026-10-13-20-08-59.gh-issue-159338.wPthPr.rst

diff --git a/Misc/NEWS.d/next/Tools-Demos/2026-10-13-20-08-59.gh-issue-159338.wPthPr.rst b/Misc/NEWS.d/next/Tools-Demos/2026-10-13-20-08-59.gh-issue-159338.wPthPr.rst
new file mode 100644
index 000000000000000..c2c22559b171f17
--- /dev/null
+++ b/Misc/NEWS.d/next/Tools-Demos/2026-10-13-20-08-59.gh-issue-159338.wPthPr.rst
@@ -0,0 +1,2 @@
+Fix ``Tools/build/check_warnings.py`` to work when ``--path-prefix``
+is omitted.