float and complex forcibly change invariant types to covariant

[jorenham]

The following example illustrates the issue, and how fixing it will give rise to an important use-case.

mypy playground

from typing import Protocol


class Just[T](Protocol):
    """
    Taken from `optype.typing.Just`.
    https://github.com/jorenham/optype?tab=readme-ov-file#just-types
    """
    @property  # type: ignore[override]
    def __class__(self, /) -> type[T]: ...
    @__class__.setter
    def __class__(self, t: type[T], /) -> None: ...


def assert_object(x: Just[object], /) -> object:
    # free sentinels anyone?
    assert type(x) is object
    return x

def assert_int(x: Just[int], /) -> int:
    # resolves python/mypy#8363
    assert type(x) is int
    return x

def assert_float(x: Just[float], /) -> float:
    # `int` promotion workaround?
    assert type(x) is float
    return x
    
    
assert_object(object())  # ok: <accepted>
assert_object(0.0)       # ok: incompatible type "float"
assert_object(-1)        # ok: incompatible type "int"
assert_object(not 1)     # ok: incompatible type "bool"

assert_int(object())     # ok: incompatible type "object"
assert_int(22 / 7)       # ok: incompatible type "float"
assert_int(0x1337)       # ok: <accepted>
assert_int(0 > -0)       # ok: incompatible type "bool"  (just let that sink in(t)...)

assert_float(object())   # ok: incompatible type "object"
assert_float(3.14)       # ok: <accepted>
assert_float(-1)         # !?: <accepted>
assert_float(False)      # !?: <accepted>

# ... `float` forcibly changes `~T@Just` to `+T@Just`?

# Some notes:
#
# * same issue with `builtins.complex`
# * user-defined types such as `class A: ...` don't have this problem
# * pyright (1.1.390) doesn't have this issue

^{huh, what's going on with the syntax highlighting?}

[jorenham]

@KotlinIsland tracked down the responsible code

mypy/mypy/subtypes.py

Lines 502 to 508 in ec4ccb0

	if not self.subtype_context.ignore_promotions:
	for base in left.type.mro:
	if base._promote and any(
	self._is_subtype(p, self.right) for p in base._promote
	):
	type_state.record_subtype_cache_entry(self._subtype_kind, left, right)
	return True