mypy tries to infer the type parameters of a generic class even if they are explicitly given

Bug Report

mypy tries to infer the type parameters of a generic class even if they are explicitly given.

To Reproduce

from typing import overload


class Frobnicator[T]:  # Using a TypeVar gives identical results
    @overload
    def __init__(self, param: T) -> None: ...
    @overload  # Removing `param` in this case gives identical results
    def __init__(self: 'Frobnicator[None]', param: None = None) -> None: ... 
    def __init__(self, param: T | None = None) -> None:
        ...

Expected Behavior

I expect the following to produce a "missing argument" error (like Pyright 1.1.384 does).

f = Frobnicator[int]()

Actual Behavior

reveal_type(f) shows Frobnicator[None]

Your Environment

The behavior is identical for mypy v1.11.1, v1.11.2 and the master branch, with both Python 3.12 and 3.13. I tried it on my local Arch + pyenv based setup and, using TypeVar, in https://mypy-play.net

Start by running the supplied Frobnicator example on the current mypy master branch and compare its result with the expected missing-argument error. Trace the type inference and overload checking involved in the explicit Frobnicatorint call; done means the call reports the missing argument instead of inferring Frobnicator[None].