gh-159338: Default the warning checker's optional path prefix to an empty string

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()

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.

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",
     )