Incompatible singledispatch signature not detected

This generates no error from mypy but fails at runtime:

from functools import singledispatch

@singledispatch
def f(x: object) -> int:
    return 1

@f.register
def _(x: int, y: str) -> int:
    return 2

print(f(1))  # No error reported, but fails at runtime

Another example:

from functools import singledispatch

@singledispatch
def f(x: object, y: str) -> None:
    y + ''

@f.register
def _(x: int, y: int) -> None:
    y + 1

print(f(1, 'x'))  # No error reported, but fails at runtime

Third example:

from functools import singledispatch

@singledispatch
def f(x: object, y: str) -> None:
    pass

@f.register
def _(x: int, yy: str) -> None:
    pass

print(f(1, y='x'))  # No error reported, but fails at runtime

We should check that the signatures of registered items are compatible with the generic one, except for the first argument.

Start with the three singledispatch examples and trace mypy's handling of registered functions and signature compatibility. Add regression coverage for positional and keyword calls, then verify that incompatible registered signatures produce errors while compatible ones remain accepted.