False negative in isinstance(var, x) when x have x.__instancecheck__

[denballakh]

from typing import Any
class X:
    def __instancecheck__(cls, x: Any) -> bool:
        return False
x: X = X()
var: Any = 123
if isinstance(var, x): # Argument 2 to "isinstance" has incompatible type "X"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"
    reveal_type(var) # Any
else:
    reveal_type(var) # Any

Expected Behavior

No errors. First reveal_type should reveal (I honestly don't know what should be here, maybe "x" or something like "InstanceOf[X]"?), second - Any

Actual Behavior
Argument 2 to "isinstance" has incompatible type "X"; expected "Union[huge union]" error. Both reveal_type's reveals Any
At runtime i get no error.

My Environment

• mypy 0.961 (compiled: no)
• no command-line flags, no config file
• CPython 3.10.4
• Windows 10

[AlexWaygood]

Duplicate of #12974