--py312-plus: type ... aliases

[asottile]

-IntOrStr: TypeAlias = int | str
+type IntOrStr = int | str

from PEP 695

[asottile]

really annoying part of the PEP that makes this not easily auto-rewritable:

Type alias expressions are not allowed to use traditional type variables (i.e. those allocated with an explicit TypeVar constructor call). Type checkers should generate an error in this case.

T = TypeVar("T")
type MyList = list[T] # Type checker error: traditional type variable usage

but maybe we just do it anyway and the user can deal? shrugs

[asottile]

it's also unclear to me when quoted forward references would ever be needed inside them. my reading of the PEP seems to indicate they are allowed but completely unnecessary... unclear.

if they're unnecessary -- the PEP 563 rewriter should apply to these as well

[asottile]

also perhaps even to the TypeVar bounds -- I've currently made them only rewrite if __future__.annotations is present -- but my reading of the PEP seems to mirror my thoughts in #836 (comment) as well so maybe this can be done unconditionally

[asottile]

wellp, this seems to be poorly specced because it breaks code that uses the type alias at runtime

for instance:

>>> from typing import TypeAlias
>>> a: TypeAlias = int
>>> isinstance(1, a)

but after rewrite it would not work at all:

>>> type a = int
>>> isinstance(1, a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: isinstance() arg 2 must be a type, a tuple of types, or a union

:(