Expression `x == y in z` crashes mypy

[p-sawicki]

Crash Report

The expression x == y in z crashes when used in assert and if statements (examples below).

I have bisected it to b91f53c @hauntsaninja

It doesn't crash anymore when I add parenthesis to make it either (x == y) in z or x == (y in z).
Traceback

/Users/psawicki/test.py:6: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.20.0+dev.b91f53c1b2d4cd12bf4e15781e447ebdd72f2b42
Traceback (most recent call last):
  File "/Users/psawicki/Code/mypy/venv3.13/bin/mypy", line 7, in <module>
    sys.exit(console_entry())
  File "/Users/psawicki/Code/mypy/mypy/__main__.py", line 15, in console_entry
    main()
  File "/Users/psawicki/Code/mypy/mypy/main.py", line 135, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/Users/psawicki/Code/mypy/mypy/main.py", line 219, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 351, in build
    result = build_inner(
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 448, in build_inner
    graph = dispatch(sources, manager, stdout)
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 3214, in dispatch
    process_graph(graph, manager)
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 3625, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 1101, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 3789, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/Users/psawicki/Code/mypy/mypy/build.py", line 2579, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 531, in check_first_pass
    self.accept(d)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 642, in accept
    stmt.accept(self)
  File "/Users/psawicki/Code/mypy/mypy/nodes.py", line 1004, in accept
    return visitor.visit_func_def(self)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 1199, in visit_func_def
    self._visit_func_def(defn)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 1203, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 1238, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 1511, in check_func_def
    self.accept(item.body)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 642, in accept
    stmt.accept(self)
  File "/Users/psawicki/Code/mypy/mypy/nodes.py", line 1582, in accept
    return visitor.visit_block(self)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 3160, in visit_block
    self.accept(s)
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 642, in accept
    stmt.accept(self)
    ~~~~~~~~~~~^^^^^^
  File "/Users/psawicki/Code/mypy/mypy/nodes.py", line 1795, in accept
    return visitor.visit_assert_stmt(self)
           ~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 5138, in visit_assert_stmt
    true_map, else_map = self.find_isinstance_check(s.expr)
                         ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 6354, in find_isinstance_check
    if_map, else_map = self.find_isinstance_check_helper(
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        node, in_boolean_context=in_boolean_context
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 6455, in find_isinstance_check_helper
    return self.comparison_type_narrowing_helper(node)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 6626, in comparison_type_narrowing_helper
    if_map, else_map = self.narrow_type_by_equality(
                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
        "==",
        ^^^^^
    ...<3 lines>...
        narrowable_indices={0},
        ^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/Users/psawicki/Code/mypy/mypy/checker.py", line 6757, in narrow_type_by_equality
    typ = get_proper_type(operand_types[i])
                          ~~~~~~~~~~~~~^^^
IndexError: list index out of range

To Reproduce

def assert_has_key(has_key: bool, key: str, s: set[str]) -> None:
    assert has_key == key in s

or

def compare_has_key(has_key: bool, key: str, s: set[str]) -> None:
    if has_key == key in s:
        print("test")

Your Environment

• Mypy version used: 1.20.0+dev.b91f53c1b2d4cd12bf4e15781e447ebdd72f2b42 and up
• Mypy command-line flags: mypy ~/test.py --show-traceback
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.13.5
• Operating system and version: macOS 15.7.3

[ilevkivskyi]

Setting priority to high, since this is a recent regression.

[hauntsaninja]

Thanks, this is a dumb bug of mine, fixing...

[hauntsaninja]

Note if this comes from real code it is probably buggy, since it desugars to (x == y and y in z). --strict-equality should catch this, and once I am done with my changes --warn-unreachable will catch it too

[hauntsaninja]

Okay I fixed the crash.

And 20660 is the PR I had planned in my stack that improves --warn-unreachable on this code. Just rebased it: https://github.com/python/mypy/pull/20660/files#diff-0413401ad6a91bc5c66579455009ba606ee0ddfb0a78278c9e53618c83b53ed5R3028