Mypy fails to infer the type of the decorated function used in the decorator

Hi mypy team,

Bug Report
I'm looking into the issue with design-by-contract library icontract which heavily uses decorators to formalize contracts. For many functions and methods, the contracts involve the decorated function (or method) itself.

See the example from the issue https://github.com/Parquery/icontract/issues/243:

from icontract import ensure

@ensure(
    lambda a, b, result: 
    result == myadd(b, a), 
    "Commutativity violated!"
)
def myadd(a: int, b: int) -> int:
    return a + b

Mypy 0.960 says:

tests_3_10\deleteme.py:5: error: Cannot determine type of "myadd"

If I add # type: ignore:

from icontract import ensure

@ensure(
    lambda a, b, result: 
    result == myadd(b, a),   # type: ignore
    "Commutativity violated!"
)
def myadd(a: int, b: int) -> int:
    return a + b

mypy does not complain any more.

I was actually a bit surprised that mypy tried to infer the types in the lambda. @claudio-ebel tried a couple of fixes using casting (see his comment https://github.com/Parquery/icontract/issues/243#issue-1229622746), but none of them worked.

I am actually not sure if this is a misunderstanding on our part or a bug in mypy. Any pointers about how to resolve the issue are very much appreciated! Please let us know if you need more information, or if there is anything we can do to help to fix this.

Your Environment

• Mypy version used: 0.960
• Mypy command-line flags: `--strict``
• Python version used: 3.10.0

Inizia riproducendo l’esempio icontract fornito con mypy 0.960 e --strict, quindi traccia i punti di ingresso dell’inferenza dei tipi del decoratore e della lambda coinvolti dall’errore. Confronta il comportamento con e senza il commento type: ignore; il lavoro è completato quando viene determinato se la diagnostica è prevista oppure l’inferenza viene corretta in modo che l’esempio venga verificato senza tale soppressione.