Callable type anntoation written apart of assignment causes false positive typecheck result.

[Renegatto]

A bugreport, mypy version is 0.761, python version is 3.7.3

This is a case where expected incapatible types, because i declared that foo's input type is float, and attempting to call it with str, but typechecking passed successful.

from typing import Callable

def bar(x):  return 5

foo : Callable[[float],int]
foo = bar

foo("test")

mypy this_file.py Mypy's output:

Success: no issues found in 1 source file

A case with type annotation written with assignment wont give false positive result:

from typing import Callable

def bar(x):  return 5

foo : Callable[[float],int] = bar

foo("test")

mypy this_file.py Mypy's output:

this_file.py:7: error: Argument 1 has incompatible type "str"; expected "float" Found 1 error in 1 file (checked 1 source file)

I wasnt found other cases excluding cases with Callable type.

For example with Tuples type anntoation written apart of assignment all working fine:

from typing import Tuple

foo : Tuple[str,int]
foo = (1,3)

mypy this_file.py Mypy's output:

this_file.py:4: error: Incompatible types in assignment (expression has type "Tuple[int, int]", variable has type "Tuple[str, int]") Found 1 error in 1 file (checked 1 source file)

[msullivan]

If any part of this is wrong, it is that part 1 doesn't result in an error. The type of foo there gets refined to def (x: Any) -> Any, which seems wrong to me, since we mostly don't do that with other types?

Our exact behaviors regarding Any and type refinement are sort of unclear and pretty heuristic

[msullivan]

@JukkaL thoughts?

[JukkaL]

The way type refinement works is inconsistent. Deciding what's the best fix is non-trivial however, since we don't want generate too many new false positives or negatives.

8540 is related (and there are probably other related issues).