Covariance error inferred for lambda has invalid line number associated with it

[kprzybyla]

Versions

mypy 0.770
python 3.8.2

Description

This is probably related to the #4270

Problem is triggered when using recipe everything_except from hypothesis documentation:

https://hypothesis.readthedocs.io/en/latest/data.html#hypothesis.strategies.from_type

I have a file with following content (this is the whole file):

from typing import TypeVar

from hypothesis import strategies as st

_Ex = TypeVar("_Ex", covariant=True)


def everything_except(*types: type) -> st.SearchStrategy[_Ex]:
    strategy = st.from_type(type).flatmap(st.from_type)

    return strategy.filter(lambda x: not isinstance(x, types))  # type: ignore

Running mypy on first clean run (without .mypy_cache) has the following output:

example.py:1276: error: Cannot use a covariant type variable as a parameter
Found 1 error in 1 file (checked 1 source file)

For some reason, mypy sees error on line 1276 which does not exist. On the other hand, running mypy for the second time gives:

Success: no issues found in 1 source file

What's more, when you have pretty set to True, first run gives:

Traceback (most recent call last):
  File "/home/krprzyby/.local/bin/mypy", line 8, in <module>
    sys.exit(console_entry())
  File "/home/krprzyby/.local/lib/python3.8/site-packages/mypy/__main__.py", line 8, in console_entry
    main(None, sys.stdout, sys.stderr)
  File "mypy/main.py", line 89, in main
  File "mypy/build.py", line 180, in build
  File "mypy/build.py", line 249, in _build
  File "mypy/build.py", line 2649, in dispatch
  File "mypy/build.py", line 2956, in process_graph
  File "mypy/build.py", line 3073, in process_stale_scc
  File "mypy/errors.py", line 476, in file_messages
  File "mypy/errors.py", line 449, in format_messages
IndexError: list index out of range

Running mypy for the second time works correctly with pretty set to True because the line number is correct and # type: ignore works.

[kprzybyla]

Also, for anyone that faces similar issue when using this recipe from hypothesis documentation, here's a working solution that avoids any mypy errors and do not require # type: ignore

from typing import TypeVar

from hypothesis import strategies as st

_T = TypeVar("_T")
_Ex = TypeVar("_Ex", covariant=True)

def everything_except(*types: type) -> st.SearchStrategy[_Ex]:
    def filter_everything_except(value: _T) -> bool:
        return not isinstance(value, types)

    return st.from_type(type).flatmap(st.from_type).filter(filter_everything_except)

[ilevkivskyi]

Wrong line number for error is a pretty serious issue, hard to say what is a possible root cause however.

[pranavrajpal]

I think the problem is that the check for whether a covariant type variable is used as the argument to a function reports the line number that the type variable is bound to a variable (which makes sense when that type variable is an argument for a FuncDef but doesn't make sense for a Callable variable or a lambda).

The wrong line number (which was line 884 in my case) seems to be pointing here.

I managed to create a somewhat simpler example:

from typing import Any, Callable, TypeVar

Ex = TypeVar("Ex")
T = TypeVar("T", covariant=True)

def takes_lambda(func: Callable[[Ex], Any]) -> None:
    pass

def test(a: T) -> None: # E: Cannot use a covariant type variable as a parameter
    func: Callable[[T], Any]
    takes_lambda(func) # type: ignore