Crash due to decorator and generator interaction

[semohr]

Crash Report

This crash likely arises due to a combination of decorators and generators. I created a minimal example below (it is derived from a more complex project and might make no sense in the shown context but the crash still occurs).

Traceback

Traceback (most recent call last):
  File "mypy/checker.py", line 576, in accept
  File "mypy/nodes.py", line 923, in accept
  File "mypy/checker.py", line 5136, in visit_decorator
  File "mypy/checker.py", line 5163, in visit_decorator_inner
  File "mypy/checkexpr.py", line 1572, in check_call
  File "mypy/checkexpr.py", line 1757, in check_callable_call
  File "mypy/checkexpr.py", line 2100, in infer_function_type_arguments
  File "mypy/checkexpr.py", line 2235, in infer_function_type_arguments_pass2
  File "mypy/infer.py", line 57, in infer_function_type_arguments
  File "mypy/constraints.py", line 251, in infer_constraints_for_callable
  File "mypy/constraints.py", line 319, in infer_constraints
  File "mypy/constraints.py", line 427, in _infer_constraints
  File "mypy/types.py", line 2009, in accept
  File "mypy/constraints.py", line 1182, in visit_callable_type
  File "mypy/constraints.py", line 1349, in infer_against_any

To Reproduce

# example.py
from enum import Enum
from typing import Callable, Generator, TypeVar, TypeVarTuple

Arg = TypeVarTuple("Arg")
Ret = TypeVar("Ret")
task = TypeVar("task")


def dec1(e: str):
    def decorator(func: Callable[[*Arg], Ret]) -> Callable[[*Arg], Ret]:

        def fn(*args: *Arg) -> Ret:
            return func(*args)

        return fn

    return decorator


def to_coro(func: Callable[[*Arg, task], Ret | task]):
    def coro(*args: *Arg) -> Generator[Ret | None | task, task, None]:
        t: None | task | Ret = None
        while True:
            t = yield t
            t = func(*(args + (t,)))

    return coro


@to_coro
@dec1("asd")
def example_function(a: int, b: int) -> int:
    return a + b

mypy ./exampe.py

If I remove the dec1 the crash disappears.

My Environment

• Mypy version used: mypy 1.15.0 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.11

I reproduced this with mypy version 1.14

[ilevkivskyi]

I have a fix ready, will make a PR soon. Couple comments in the meantime:

• Your decorators are missing return types, in mypy a missing function annotation (whether return or argument) always means Any. This means those decorators "destroy" the types of functions they decorate (we have options to warn about such cases, like --disallow-untyped-decorators, or even stricter one --disallow-any-decorated).
• Although not technically an error, usually a union of two plain type variables is a "bad" type, see if you can refactor your real code to avoid this.

[semohr]

Thanks for the fast fix!
I will have a look to incorporate --disallow-untyped-decorators. I think for the my real code this should already be properly implemented.