ParamSpec erases Optional when no-strict-optional is enabled

[koogoro]

Bug Report

The latest master will cause ParamSpec to replace parameters with type Optional[T] with type T when in a file with no-strict-optional. This is different from the behavior on 1.5.1. While this is equivalent within the same file, if a function affected by this issue is imported to a file without no-strict-optional, mypy will no longer let you pass None into that parameter. This behavior affects decorators such as contextlib.contextmanager.

To Reproduce

https://gist.github.com/mypy-play/dc17766e33ca6f4a1ec440257b0462ac

Expected Behavior

main.py:10: note: Revealed type is "def (arg: Union[builtins.int, None])"
Success: no issues found in 1 source file

Actual Behavior

main.py:10: note: Revealed type is "def (arg: builtins.int)"
Success: no issues found in 1 source file

Your Environment

I bisected the commits and found that this issue first occurs on 14418bc.

[ilevkivskyi]

This is consistent with behavior for regular TypeVars (since at least 1.0.0, but likely it is much older):

from typing import Optional, TypeVar
from typing import Callable

def a(arg: Optional[int]) -> None: ...

T = TypeVar('T')
def b(f: Callable[[T], None]) -> Callable[[T], None]: ...

reveal_type(b(a))  # Revealed type is "def (builtins.int)"

Unions were simplified during expansion for ages, and inconsistent behavior for ParamSpec was a bug.