Mypyc gcc compilation fails with specific duplicate variable definition

[CoolCat467]

Crash Report

Using mypyc to compile simple.py (slimmed down example of real use-case wherein I accidentally set typed variable twice in different ways) leads to compilation failure with GCC instead of mypy noticing something is amiss.

simple.py

from __future__ import annotations

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from mypy_extensions import u8


def next_down(
    depth: u8 = 5,
) -> u8:
    next_down = depth - 1

    ##### KEY TO FAILURE #####
    next_down = None if depth is None else depth - 1
    ##########################
    return next_down


if __name__ == "__main__":
    print(next_down())

mypyc simple.py

Traceback

/home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/setuptools/config/_apply_pyprojecttoml.py:82: SetuptoolsDeprecationWarning: `project.license` as a TOML table is deprecated
!!

        ********************************************************************************
        Please use a simple string containing a SPDX expression for `project.license`. You can also use `project.license-files`. (Both options available on setuptools>=77.0.0).

        By 2026-Feb-18, you need to update your project and remove deprecated calls
        or your builds will no longer be supported.

        See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
        ********************************************************************************

!!
  corresp(dist, value, root_dir)
running build_ext
building 'package_name.simple__mypyc' extension
x86_64-linux-gnu-gcc -fno-strict-overflow -Wsign-compare -DNDEBUG -g -O2 -Wall -fPIC -I/home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/mypyc/lib-rt -Ibuild -I/home/<username>/Desktop/mypyc-crash/.venv/include -I/usr/include/python3.13 -c build/package_name/__native_simple.c -o build/temp.linux-x86_64-cpython-313/build/package_name/__native_simple.o -O3 -g1 -Werror -Wno-unused-function -Wno-unused-label -Wno-unreachable-code -Wno-unused-variable -Wno-unused-command-line-argument -Wno-unknown-warning-option -Wno-unused-but-set-variable -Wno-ignored-optimization-argument -Wno-cpp
In file included from /home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/mypyc/lib-rt/init.c:2,
                 from build/package_name/__native_simple.c:1:
In function ‘CPyLong_AsUInt8’,
    inlined from ‘CPyDef_next_down’ at build/package_name/__native_simple.c:95:16:
/home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/mypyc/lib-rt/CPy.h:396:16: error: array subscript ‘PyLongObject {aka struct _longobject}[0]’ is partly outside array bounds of ‘PyObject[1]’ {aka ‘struct _object[1]’} [-Werror=array-bounds=]
  396 |         size_t tag = CPY_LONG_TAG(lobj);
      |                ^~~
In file included from /usr/include/python3.13/Python.h:72,
                 from /home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/mypyc/lib-rt/init.c:1:
/usr/include/python3.13/object.h: In function ‘CPyDef_next_down’:
/usr/include/python3.13/object.h:1101:22: note: object ‘_Py_NoneStruct’ of size 16
 1101 | PyAPI_DATA(PyObject) _Py_NoneStruct; /* Don't use this directly */
      |                      ^~~~~~~~~~~~~~
In function ‘CPyLong_AsUInt8’,
    inlined from ‘CPyDef_next_down’ at build/package_name/__native_simple.c:95:16:
/home/<username>/Desktop/mypyc-crash/.venv/lib/python3.13/site-packages/mypyc/lib-rt/CPy.h:399:19: error: array subscript ‘PyLongObject {aka struct _longobject}[0]’ is partly outside array bounds of ‘PyObject[1]’ {aka ‘struct _object[1]’} [-Werror=array-bounds=]
  399 |             digit x = CPY_LONG_DIGIT(lobj, 0);
      |                   ^
/usr/include/python3.13/object.h: In function ‘CPyDef_next_down’:
/usr/include/python3.13/object.h:1101:22: note: object ‘_Py_NoneStruct’ of size 16
 1101 | PyAPI_DATA(PyObject) _Py_NoneStruct; /* Don't use this directly */
      |                      ^~~~~~~~~~~~~~
At top level:
cc1: note: unrecognized command-line option ‘-Wno-ignored-optimization-argument’ may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option ‘-Wno-unknown-warning-option’ may have been intended to silence earlier diagnostics
cc1: note: unrecognized command-line option ‘-Wno-unused-command-line-argument’ may have been intended to silence earlier diagnostics
cc1: all warnings being treated as errors
error: command '/usr/bin/x86_64-linux-gnu-gcc' failed with exit code 1

To Reproduce

See above

Your Environment

• Mypy version used: mypy 1.17.1 (compiled: yes)
• Mypy command-line flags: See above
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.13.3 (main, Jun 16 2025, 18:15:32) [GCC 14.2.0] on linux
• Operating system and version:

> uname -a
Linux ideapad 6.14.0-24-generic #24-Ubuntu SMP PREEMPT_DYNAMIC Sun Jun 15 11:18:07 UTC 2025 x86_64 x86_64 x86_64 GNU/Linux

• GCC version

> x86_64-linux-gnu-gcc --version
x86_64-linux-gnu-gcc (Ubuntu 14.2.0-19ubuntu2) 14.2.0
...

[brianschubert]

Thanks, here's a slightly simpler reproducer:

from typing import Literal
def false() -> Literal[False]: return False

None if false() else 123

Seems to only reproduce when the true branch of the ternary is unreachable and None-valued.

[CoolCat467]

Not sure how to access with mypyc CLI, but noticed that this only happens if opt_level >= 2 (mypycify call) (or at least that I can remember, haven't tested on minimum version, only original that this is based on)

Edit: Tested, indeed correct, build does not fail if opt_level is 1