From a623addca666938de4c6a812e0d8f7456ed80b94 Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Wed, 12 Aug 2020 13:24:17 +0200
Subject: [PATCH] bpo-41520: codeop no longer ignores SyntaxWarning

---
 Lib/codeop.py                                              | 6 ++++--
 Lib/test/test_codeop.py                                    | 7 +++++++
 .../next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst  | 1 +
 3 files changed, 12 insertions(+), 2 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst

diff --git a/Lib/codeop.py b/Lib/codeop.py
index 7e192ea6a10a03..547629262d0689 100644
--- a/Lib/codeop.py
+++ b/Lib/codeop.py
@@ -84,9 +84,11 @@ def _maybe_compile(compiler, source, filename, symbol):
     except SyntaxError:
         pass
 
-    # Suppress warnings after the first compile to avoid duplication.
+    # Catch syntax warnings after the first compile
+    # to emit SyntaxWarning at most once.
     with warnings.catch_warnings():
-        warnings.simplefilter("ignore")
+        warnings.simplefilter("error", SyntaxWarning)
+
         try:
             code1 = compiler(source + "\n", filename, symbol)
         except SyntaxError as e:
diff --git a/Lib/test/test_codeop.py b/Lib/test/test_codeop.py
index 6e821df6b0e707..7984e5f1e5a69a 100644
--- a/Lib/test/test_codeop.py
+++ b/Lib/test/test_codeop.py
@@ -4,6 +4,7 @@
 """
 import sys
 import unittest
+import warnings
 from test import support
 from test.support import warnings_helper
 
@@ -310,5 +311,11 @@ def test_warning(self):
             compile_command("0 is 0")
             self.assertEqual(len(w.warnings), 1)
 
+        # bpo-41520: check SyntaxWarning treated as an SyntaxError
+        with self.assertRaises(SyntaxError):
+            warnings.simplefilter('error', SyntaxWarning)
+            compile_command('1 is 1\n', symbol='exec')
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst b/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst
new file mode 100644
index 00000000000000..ca5501c2aeec0d
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-08-12-13-25-16.bpo-41520.BEUWa4.rst
@@ -0,0 +1 @@
+Fix :mod:`codeop` regression: it no longer ignores :exc:`SyntaxWarning`.