Type inference for protocol is confused with keyword-only, reordered arguments

[Elscouta]

• Are you reporting a bug, or opening a feature request?

I'm reporting a bug.

• Please insert below the code you are checking with mypy,

from typing import TypeVar
from typing_extensions import Protocol

TA = TypeVar("TA", contravariant=True)

class P(Protocol[TA]):
    def __call__(self, *, a: TA, b: int) -> None: ...

def misordered(*, b: int, a: TA) -> None: ... # Should be seen as Protocol[TA]
def ordered(*, a: TA, b: int) -> None: ...      # Should be seen as Protocol[TA]
def explicit(*, b: int, a: str) -> None: ...    # Should be seen as Protocol[str]
    
    
def supports_protocol_int(a: P[int]) -> None: ... 
def supports_protocol_str(a: P[str]) -> None: ...
    
supports_protocol_int(misordered) # Works, since TA = int does not cause problem
supports_protocol_str(misordered) # BUG: Does not work. TA is determined to be int.
supports_protocol_int(ordered)    # Works.
supports_protocol_str(ordered)    # Works.
supports_protocol_int(explicit)   # Returns an error (as expected)
supports_protocol_str(explicit)   # Works.

• What is the actual behavior/output?

main.py:18: error: Argument 1 to "supports_protocol_str" has incompatible type "Callable[[NamedArg(int, 'b'), NamedArg(TA, 'a')], None]"; expected "P[str]"
main.py:21: error: Argument 1 to "supports_protocol_int" has incompatible type "Callable[[NamedArg(int, 'b'), NamedArg(str, 'a')], None]"; expected "P[int]"
Found 2 errors in 1 file (checked 1 source file)

• What is the behavior/output you expect?

Only the second error is correct.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

This was tested on the "real code" with mypy-0.760 and mypy-0.720. The minimal example was run on mypy-play.net, mypy latest, as of 23/01/2020.

• What are the mypy flags you are using? (For example --strict-optional)

No special settings were done on mypy-play.net

[ilevkivskyi]

Yeah, looks like a bug to me. Order of keyword-only arguments should not affect type inference, only their names. Should be probably not hard to fix.

[TH3CHARLie]

Does this require to modify unify_generic_callable to support unify generic by keyword?