Fix subtyping between ParamSpecs

[ilevkivskyi]

Fixes #14169
Fixes #14168

Two sings here:

• Actually check prefix when we should
• strict_concatenate check should be off by default (IIUC it is not mandated by the PEP)

cc @A5rocks

[ilevkivskyi]

And now this error removed in my previous PR is back, LOL.

[A5rocks]

Quick comment since I haven't looked over the code yet:

strict_concatenate check should be off by default (IIUC it is not mandated by the PEP)

My impression (I forgot everything...) is that it's mandated. But now I looked back at the PEP and found this example, which is not allowed by strict concatenate:

def add(f: Callable[P, int]) -> Callable[Concatenate[str, P], None]:

  def foo(s: str, *args: P.args, **kwargs: P.kwargs) -> None:  # Accepted
    pass

[snip]

  return foo                                                   # Accepted

[A5rocks]

The logic makes sense to me. Not entirely sure why you switched the default but changing that's fine.

[A5rocks]

I notice you use Protocol instead of Generic. While this should not care about that (obviously), mind adding one or shifting a test case over?

[ilevkivskyi]

I will add one test case.

[ilevkivskyi]

Not entirely sure why you switched the default but changing that's fine.

Without it one test case fails. Also it is a deviation from the PEP, so probably should not be on by default. (Btw I noticed in one test case you added: "this is one noticeable deviation from PEP but I believe it is for the better", so I guess it was an intentional decision).

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

discord.py (https://github.com/Rapptz/discord.py)
+ discord/ext/commands/core.py:1799: error: Overloaded function signatures 1 and 2 overlap with incompatible return types  [misc]