Allow PEP 604 type aliases to be used in runtime context.

[esoma]

I ramble about this here #11648 😄

This only solves the main issue that I had with #11648 (that a metaclass __or__ return type was lost in mypy when giving it a name).

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This does not address the issue of union type aliases not being use-able with isinstance:

class A: ...
class B: ...
C = A | B
reveal_type(C) # Revealed type is "builtins.object"
isinstance(object(), C) # error: Parameterized generics cannot be used with class or instance checks
                        # error: Argument 2 to "isinstance" has incompatible type "object"; expected "Union[type, UnionType, Tuple[Union[type, UnionType, Tuple[Any, ...]], ...]]"

But it does get rid of the second error regarding argument 2 not being compatible and correctly reveals C as types.UnionType.

[sobolevn]

Thank you for your work! 🎉

I have left several questions 🙂

[sobolevn]

I haven't personally used this trick yet, so I am thinking 🤔
Should not this be just F? Or do we want this special treatment of types.UnionType?

[esoma]

I think there is confusion over Union and UnionType here (and imo, it is confusing and that's because unions are treated so specially). UnionType is the runtime type of a Union object:

X = str | int
print(type(X)) # <class 'types.UnionType'>

So, the type of X is always UnionType (it's worth noting that in previous version of mypy this was just builtins.object).

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This is perhaps easier to grasp given:

class A: ...
reveal_type(A | A)  # types.UnionType
a: A | A
reveal_type(a) # Union[test2.A, test2.A]

Now, the second case not reducing is almost certainly a bug, but I think that is unrelated to this specific issue (it's also present in master) since it doesn't matter if its a TypeAlias or not (just if it was a PEP 604 union or not).

[brent-moffit]

It's been a while now, but this PR would still be useful. In the meantime I've worked around this using cast:

class A: ...
class B: ...
C = A | B
isinstance(object(), cast(UnionType, C))

[hauntsaninja]

I think these all work on master with sufficiently new Python