Unable to overload __new__ method with TypeVar input

Bug Report

Unable to overload new method with generic input

To Reproduce
How can I type hint my __new__ implementation using a generic input?

So I have a class that can store str or decimal.Decimal instances, and it subclasses the input str or decimal.Decimal class.

This code works in mypy and python:

from __future__ import annotations
import typing
import typing_extensions
import decimal
import random

T = typing.TypeVar('T', str, decimal.Decimal)

class StrOrDecimal(typing.Generic[T]):
    @typing.overload
    def __new__(cls, arg: decimal.Decimal) -> StrOrDecimalTyped.Decimal.StrOrDecimal[decimal.Decimal]:
        ...
    @typing.overload
    def __new__(cls, arg: str) -> StrOrDecimalTyped.Str.StrOrDecimal[str]:
        ...
    # def __new__(cls, arg: typing.Union[str, decimal.Decimal]): does not work
    def __new__(cls, arg):  # works
        if isinstance(arg, decimal.Decimal):
            return super(cls, StrOrDecimalTyped.Decimal.StrOrDecimal).__new__(StrOrDecimalTyped.Decimal.StrOrDecimal, arg)
        if isinstance(arg, str):
            return super(cls, StrOrDecimalTyped.Str.StrOrDecimal).__new__(StrOrDecimalTyped.Str.StrOrDecimal, arg)
        raise ValueError('Invalid value')

_StrOrDecimal = StrOrDecimal
# needed for mypy
class StrOrDecimalTyped:
    class Str:
        class StrOrDecimal(_StrOrDecimal[T], str):
            pass
    class Decimal:
        class StrOrDecimal(_StrOrDecimal[T], decimal.Decimal):
            pass

a_val = 'a'
a = StrOrDecimal(a_val)
print(a)

b_val = decimal.Decimal('1.2')
b = StrOrDecimal(b_val)
print(b)

choices: typing.List[typing.Union[str, decimal.Decimal]] = [a_val, b_val]
c_val = random.choice(choices)
c = StrOrDecimal(c_val)

try:
    reveal_type(a)
    reveal_type(b)
    reveal_type(c)
except NameError:
    typing_extensions.reveal_type(a)
    typing_extensions.reveal_type(b)
    typing_extensions.reveal_type(c)

But if I use either of these signatures:

def __new__(cls, arg: typing.Union[str, decimal.Decimal]):
def __new__(cls, arg: T):

I get this error:

generic.py:19: error: Argument 1 to "__new__" of "StrOrDecimal" has incompatible type "Type[generic.StrOrDecimalTyped.Decimal.StrOrDecimal[Any]]"; expected "Type[generic.StrOrDecimal[T]]"

How can I fix it?

Expected Behavior

I expect to be able to use T as the input type

Actual Behavior

Mypy throws the above error

Your Environment

• Mypy version used:
• Mypy command-line flags:
• Mypy configuration options from mypy.ini (and other config files):
• Python version used:

Start by running the provided reproduction in generic.py and inspect the reported error at line 19, focusing on how mypy checks the new signature with TypeVar and overloads. Compare the overload, Union, and TypeVar variants; done means a supported generic input form type-checks without the incompatible-argument error while retaining the expected revealed types.