fix UnionType.__or__ and add UnionType.__getitem__

[KotlinIsland]

UnionType.__or__/__ror__

from types import UnionType
from typing import _SpecialForm

def f(u: UnionType, s: _SpecialForm):
    x = u | s
    reveal_type(x)  # UnionType, is actually _SpecialForm (is actually _UnionGenericAlias)
f(int | str, Literal[1])

this is rather unfortunate due to NotImplemented semantics, which are underspecified at this time, very relevant:

• special case Any in righthand-able operators to be NotImplemented lenient DetachHead/basedpyright#1465

to work around this we add the known case _SpecialForm to the return type

and

x = int | int | int
reveal_type(x) # UnionType, is actually type[int]

to work around this we add the special case type to the return type, in the case of "unions are invalid as return types", then i would suggest UnionType | Any

UnionType.__getitem__

T = TypeVar("T")
A = int | list[T]

A[int]

it has __getitem__, the design was inspired by _SpecialForm.__getitem__ but suggestions are appreciated

[srittau]

@JelleZijlstra Maybe you could have a look as resident UnionType expert?

[AlexWaygood]

I don't understand this comment.

The way we approach this for every other BinOp dunder in typeshed is by annotating the parameters for the reflected dunder on the r.h.s. appropriately. For example, int.__add__ is annotated as only accepting int and only returning int, because int is the only type that can be passed to int.__add__ without int.__add__ returning NotImplemented:

typeshed/stdlib/builtins.pyi

Line 296 in a2c34cc

def __add__(self, value: int, /) -> int: ...

But float.__radd__ is annotated as accepting float (interpreted by type checkers as int | float due to the int/float special case), and it is because of this __radd__ method that type checkers are able to infer that 1 + 3.4 should be understood as evaluating to an instance of float:

typeshed/stdlib/builtins.pyi

Line 387 in a2c34cc

def __radd__(self, value: float, /) -> float: ...

Why would we do something differently here than what we do for every other BinOp dunder in typeshed?

[KotlinIsland]

the issue is that UnionType.__or__ accepts Any, so it is clobbering _SpecialForm.__ror__

[AlexWaygood]

Ah, okay, that makes sense. Could you expand the comment a bit to say that?

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉