Higher-order function is not covered by crosshair cover

[pschanely]

Expected vs actual behavior
The TIM Improves Merging project found that crosshair could not generate two inputs to cover each branch of the following function (see additional discussion in the report PDF). Here, we see only the first branch is covered:

$ cat callableex.py
from typing import Any, Dict, List, Optional, Tuple, Generator, Callable, Iterable, IO, TypeVar, Mapping

def lambdaFn(a: Callable[[int], int]):
    if a:
        return a(2) + 4
    else:
        return "hello"

$ crosshair cover callableex.lambdaFn
lambdaFn(lambda *a: None)

One might expect to see inputs that cover both paths in the if statement.

[pschanely]

(I'm self-reporting and self-answering here, for the benefit of future CrossHair users)

In order to generate None CrossHair demands that the type have the Optional[] wrapper. This might be a common sort of stumbling block - mypy only recently made --no-implicit-optional the default behavior.

Adding Optional gives us the two paths we expect:

$ cat callableex.py
from typing import Any, Dict, List, Optional, Tuple, Generator, Callable, Iterable, IO, TypeVar, Mapping

def lambdaFn(a: Optional[Callable[[int], int]]):
    if a:
        return a(2) + 4
    else:
        return "hello"
$ crosshair cover callableex.lambdaFn
lambdaFn(lambda *a: None)
lambdaFn(None)