mypyc crashes when using a narrowed type that is not found at compile time

[twoertwein]

Bug Report

The following worked with mypy<1.0.

To Reproduce

import not_found_at_compile_time
# in my case: the PEP-517 build dependencies did not list all the runtime dependencies to be also present at build/compile time

def test(x: dict[int, int] | not_found_at_compile_time.A) -> None:
    if isinstance(x, dict):
        test = {key: value for key, value in x.items()}

Expected Behavior

mypyc does not crash.

Actual Behavior

mypyc crashes:

$ mypy test.py
Success: no issues found in 1 source file

$ mypyc test.py
Traceback (most recent call last):
File "mypyc/irbuild/builder.py", line 247, in accept
File "mypy/nodes.py", line 2368, in accept
File "mypyc/irbuild/visitor.py", line 314, in visit_dictionary_comprehension
File "mypyc/irbuild/expression.py", line 995, in transform_dictionary_comprehension
File "mypyc/irbuild/for_helpers.py", line 330, in comprehension_helper
File "mypyc/irbuild/for_helpers.py", line 290, in handle_loop
File "mypyc/irbuild/for_helpers.py", line 101, in for_loop_helper
File "mypyc/irbuild/for_helpers.py", line 467, in make_for_loop_generator
File "mypyc/irbuild/builder.py", line 901, in get_dict_item_type
File "mypyc/irbuild/builder.py", line 893, in get_dict_key_type
File "mypyc/irbuild/builder.py", line 888, in get_dict_base_type
test.py:6: AssertionError: Union[builtins.dict[builtins.int, builtins.int], builtins.dict[Any, Any]]

Your Environment

• Mypy version used: 1.0.0
• Python version used: 3.11.1

[ichard26]

This is the same bug fundamentally as mypyc/mypyc#965.

[twoertwein]

Feel free to close it here if that makes it easier to manage/prioritize the issues.

I think the above example worked with the pervious mypy release.

To be fair, I somehow like the new "behavior": as it ensures that all dependencies are installed at compile time :) I assume, while previous mypyc version did not crash, they probably compiled less ideal code (key and values were probably Any instead of int).