Segfault crash on specific recursive type situation

Bug Report

Mypy segfaults when checking the following code.

To Reproduce

from typing import Self, Union

type OneClass = list[OneClass|int]

class TwoClass (list[Union['TwoClass',int]]):
    @classmethod
    def meth (cls, obj:Self|OneClass) -> Self|None:
        if isinstance(obj, cls): return obj

Actual Behavior

In terminal:

Segmentation fault         (core dumped) mypy test.py

In gist:

Failed (exit code: -11)

Your Environment

Mypy version used: mypy 1.18.2 (compiled: yes)
Mypy command-line flags: None
Mypy configuration options from mypy.ini (and other config files): None
Python version used: Python 3.13.7

Slightly simpler reproducer:

type OneClass = list[OneClass]

class TwoClass(list['TwoClass']):
    pass

def f(obj: OneClass) -> None:
    if isinstance(obj, TwoClass): pass

Traceback from the interpreted version (current master - #b266dd):

Traceback (most recent call last):
  File "/tmp/tmp.d9375bltfC/.venv/bin/mypy", line 10, in <module>
    sys.exit(console_entry())
  File "/home/stas/Documents/Work/mypy/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/stas/Documents/Work/mypy/mypy/main.py", line 127, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/stas/Documents/Work/mypy/mypy/main.py", line 211, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 213, in build
    result = _build(
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 292, in _build
    graph = dispatch(sources, manager, stdout)
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 2931, in dispatch
    process_graph(graph, manager)
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 3322, in process_graph
    done, still_working = manager.wait_for_done(graph)
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 915, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 3460, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/home/stas/Documents/Work/mypy/mypy/build.py", line 2322, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 524, in check_first_pass
    self.accept(d)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 635, in accept
    stmt.accept(self)
  File "/home/stas/Documents/Work/mypy/mypy/nodes.py", line 971, in accept
    return visitor.visit_func_def(self)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 1192, in visit_func_def
    self._visit_func_def(defn)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 1196, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 1231, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 1504, in check_func_def
    self.accept(item.body)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 635, in accept
    stmt.accept(self)
  File "/home/stas/Documents/Work/mypy/mypy/nodes.py", line 1542, in accept
    return visitor.visit_block(self)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 3127, in visit_block
    self.accept(s)
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 635, in accept
    stmt.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/nodes.py", line 1810, in accept
    return visitor.visit_if_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 5022, in visit_if_stmt
    if_map, else_map = self.find_isinstance_check(e)
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 6281, in find_isinstance_check
    if_map, else_map = self.find_isinstance_check_helper(
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        node, in_boolean_context=in_boolean_context
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 6304, in find_isinstance_check_helper
    *self.conditional_types_with_intersection(
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        self.lookup_type(expr), self.get_isinstance_type(node.args[1]), expr
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ),
    ^
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 7841, in conditional_types_with_intersection
    initial_types = conditional_types(
        expr_type,
    ...<2 lines>...
        consider_runtime_isinstance=consider_runtime_isinstance,
    )
  File "/home/stas/Documents/Work/mypy/mypy/checker.py", line 8220, in conditional_types
    elif not is_overlapping_types(current_type, proposed_type, ignore_promotions=True):
             ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 630, in is_overlapping_types
    if all(
       ~~~^
        _is_overlapping_types(left_arg, right_arg)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        for left_arg, right_arg in zip(left_args, right_args)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ):
    ^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 631, in <genexpr>
    _is_overlapping_types(left_arg, right_arg)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 433, in _is_overlapping_types
    return is_overlapping_types(
        left,
    ...<4 lines>...
        seen_types=seen_types.copy(),
    )
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 630, in is_overlapping_types
    if all(
       ~~~^
        _is_overlapping_types(left_arg, right_arg)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
        for left_arg, right_arg in zip(left_args, right_args)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ):
    ^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 631, in <genexpr>
  [... repeated many times]
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 631, in <genexpr>
    _is_overlapping_types(left_arg, right_arg)
    ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 433, in _is_overlapping_types
    return is_overlapping_types(
        left,
    ...<4 lines>...
        seen_types=seen_types.copy(),
    )
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 403, in is_overlapping_types
    if are_related_types(
       ~~~~~~~~~~~~~~~~~^
        left, right, proper_subtype=overlap_for_overloads, ignore_promotions=ignore_promotions
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ):
    ^
  File "/home/stas/Documents/Work/mypy/mypy/meet.py", line 319, in are_related_types
    return is_subtype(left, right, ignore_promotions=ignore_promotions) or is_subtype(
           ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/subtypes.py", line 148, in is_subtype
    if left == right:
       ^^^^^^^^^^^^^
  File "/home/stas/Documents/Work/mypy/mypy/types.py", line 1645, in __eq__
    self.type == other.type
RecursionError: maximum recursion depth exceeded in comparison

Remember the pair in `is_overlapping_types` if at least one of them i…

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Segfault crash on specific recursive type situation #20107

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Segfault crash on specific recursive type situation #20107

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A5rocks commented on Oct 23, 2025

sterliakov commented on Oct 23, 2025

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