gh-159340: Detect partial reductions in expected compiler warnings

Lib/test/test_tools/test_check_warnings.py

@@ -0,0 +1,46 @@
+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_partial_improvement(self):
+        for expected, actual, ignore_all, status in (
+            (2, 1, False, 1), (2, 0, False, 1),
+            (2, 2, False, 0), (2, 3, False, 0),
+            (0, 0, False, 0), (2, 1, True, 0),
+        ):
+            with self.subTest(expected=expected, actual=actual,
+                              ignore_all=ignore_all):
+                rules = {check_warnings.IgnoreRule(
+                    "source.c", expected, ignore_all)}
+                warnings = {"source.c": [{}] * actual} if actual else {}
+                with captured_stdout():
+                    result = check_warnings.get_unexpected_improvements(
+                        rules, warnings)
+                self.assertEqual(result, status)
+
+    def test_cli_partial_improvement(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 2\n", encoding="utf-8")
+            args = ["-c", str(output), "-i", str(ignore), "-t", "gcc", "-p", ""]
+            with captured_stdout():
+                self.assertEqual(check_warnings.main(args), 0)
+                self.assertEqual(check_warnings.main(
+                    args + ["--fail-on-improvement"]), 1)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Tools-Demos/2026-10-13-20-09-07.gh-issue-159340.wCntIm.rst

@@ -0,0 +1,3 @@
+Fix ``Tools/build/check_warnings.py`` to detect warning-count reductions
+when some warnings remain, and to accept zero expected warnings when none
+are present.

Tools/build/check_warnings.py

@@ -197,23 +197,19 @@ def get_unexpected_improvements(
     files_with_warnings: dict[str, list[CompileWarning]],
 ) -> int:
     """
-    Returns failure status if the number of warnings for a file is greater
+    Returns failure status if the number of warnings for a file is smaller
     than the expected number of warnings for that file based on the ignore
     rules
     """
     unexpected_improvements = []
     for rule in ignore_rules:
-        if (
-            not rule.ignore_all
-            and rule.file_path not in files_with_warnings.keys()
-        ):
-            if rule.file_path not in files_with_warnings.keys():
-                unexpected_improvements.append((rule.file_path, rule.count, 0))
-            elif len(files_with_warnings[rule.file_path]) < rule.count:
+        if not rule.ignore_all:
+            count = len(files_with_warnings.get(rule.file_path, ()))
+            if count < rule.count:
                 unexpected_improvements.append((
                     rule.file_path,
                     rule.count,
-                    len(files_with_warnings[rule.file_path]),
+                    count,
                 ))
 
     if unexpected_improvements: