bpo-39562: Prevent collision of future and compiler flags (GH-19230)

The constant values of future flags in the __future__ module
is updated in order to prevent collision with compiler flags.
Previously PyCF_ALLOW_TOP_LEVEL_AWAIT was clashing
with CO_FUTURE_DIVISION.

2 files changed, 25 insertions, 8 deletions

diff --git a/Lib/__future__.py b/Lib/__future__.py
index e1135685d8..d7cb8ac5f4 100644
--- a/Lib/__future__.py
+++ b/Lib/__future__.py
@@ -68,14 +68,14 @@ __all__ = ["all_feature_names"] + all_feature_names
 # this module.
 CO_NESTED            = 0x0010   # nested_scopes
 CO_GENERATOR_ALLOWED = 0        # generators (obsolete, was 0x1000)
-CO_FUTURE_DIVISION   = 0x2000   # division
-CO_FUTURE_ABSOLUTE_IMPORT = 0x4000 # perform absolute imports by default
-CO_FUTURE_WITH_STATEMENT  = 0x8000   # with statement
-CO_FUTURE_PRINT_FUNCTION  = 0x10000   # print function
-CO_FUTURE_UNICODE_LITERALS = 0x20000 # unicode string literals
-CO_FUTURE_BARRY_AS_BDFL = 0x40000
-CO_FUTURE_GENERATOR_STOP  = 0x80000 # StopIteration becomes RuntimeError in generators
-CO_FUTURE_ANNOTATIONS     = 0x100000  # annotations become strings at runtime
+CO_FUTURE_DIVISION   = 0x20000   # division
+CO_FUTURE_ABSOLUTE_IMPORT = 0x40000 # perform absolute imports by default
+CO_FUTURE_WITH_STATEMENT  = 0x80000   # with statement
+CO_FUTURE_PRINT_FUNCTION  = 0x100000   # print function
+CO_FUTURE_UNICODE_LITERALS = 0x200000 # unicode string literals
+CO_FUTURE_BARRY_AS_BDFL = 0x400000
+CO_FUTURE_GENERATOR_STOP  = 0x800000 # StopIteration becomes RuntimeError in generators
+CO_FUTURE_ANNOTATIONS     = 0x1000000  # annotations become strings at runtime
 
 class _Feature:
     def __init__(self, optionalRelease, mandatoryRelease, compiler_flag):
diff --git a/Lib/test/test_future.py b/Lib/test/test_future.py
index fdca2312fa..56b7ac6865 100644
--- a/Lib/test/test_future.py
+++ b/Lib/test/test_future.py
@@ -1,5 +1,7 @@
 # Test various flavors of legal and illegal future statements
 
+import __future__
+import ast
 import unittest
 from test import support
 from textwrap import dedent
@@ -75,6 +77,21 @@ class FutureTest(unittest.TestCase):
             from test import badsyntax_future10
         self.check_syntax_error(cm.exception, "badsyntax_future10", 3)
 
+    def test_ensure_flags_dont_clash(self):
+        # bpo-39562: test that future flags and compiler flags doesn't clash
+
+        # obtain future flags (CO_FUTURE_***) from the __future__ module
+        flags = {
+            f"CO_FUTURE_{future.upper()}": getattr(__future__, future).compiler_flag
+            for future in __future__.all_feature_names
+        }
+        # obtain some of the exported compiler flags (PyCF_***) from the ast module
+        flags |= {
+            flag: getattr(ast, flag)
+            for flag in dir(ast) if flag.startswith("PyCF_")
+        }
+        self.assertCountEqual(set(flags.values()), flags.values())
+
     def test_parserhack(self):
         # test that the parser.c::future_hack function works as expected
         # Note: although this test must pass, it's not testing the original