User defined type-level operators works, are they legal?

While looking for a workaround about the lack of extensible typed records, found some approaches that works, but then I realized, I'm using quite a few type level operators for that, are they legal?

My use case is too cumbersome and irrelevant to explain, but a simpler self-explanatory example:

Retrieving inner type of Awaitable:

from typing import Awaitable, Type, TypeVar

A = TypeVar("A")


class typeofMeta(type):
    def __getitem__(cls, value: A) -> Type[A]: raise NotImplementedError
class AwaitedMeta(type):
    def __getitem__(cls, value: Type[Awaitable[A]]) -> Type[A]: raise NotImplementedError

class typeof(metaclass=typeofMeta): ...
class Awaited(metaclass=AwaitedMeta): ...


async def add(a: int, b: int) -> int:
    return a + b


coro = add(1, 2)

reveal_type(Awaited[typeof[coro]])
# Revealed type is "Type[builtins.int]"

By defining a __getitem__ at class level I can do whatever with the types and make native-looking type operators. After that I noticed the use of @_SpecialForm in typing internals, but no mention that I could find in docs if this use can be replicated by developers. So I'm wondering if it's safe to rely on this kind of stuff, it enables a lot of nice composition patterns and zero-cost abstractions structures, like is common is TS.

Here are a few more typescript util clones, full implementation in gist:

Type-Level if-else

reveal_type(InstanceType[If[True, int, str]])
# Revealed type is 'builtins.int'

reveal_type(InstanceType[If[False, int, str]])
# Revealed type is 'builtins.str'

Get precise type of an object

def add(a: int, b: int) -> int: return a + b

reveal_type(typeof[add])
# Revealed type is 'def (builtins.int, builtins.int) -> builtins.int'

reveal_type(Type[add])  # with builtin `Type` get down to object
# Revealed type is builtins.object'

Get instance type from a class

class Box(Generic[A]):
    value: A

reveal_type(InstanceType[Box[int]])
# Revealed type is 'Box[builtins.int]'

Get return type of a callable

def add(a: int, b: int) -> int: return a + b

reveal_type(ReturnType[add])
# Revealed type is 'builtins.int'

Get parameter typle of a callable

def add(a: int, b: int) -> int: return a + b

reveal_type(Parameters[add])
# Revealed type is 'Tuple[builtins.int, builtins.int]'

Get inner type of an Awaitable

async def add(a: int, b: int) -> int: return a + b
coro = add(1, 2)

reveal_type(Awaited[typeof[coro]])
# Revealed type is "Type[builtins.int]"

Get item type of an iterable

reveal_type(ItemOf[list[int]])
# Revealed type is 'Type[builtins.int]'

Get key type of a Mapping

reveal_type(keyof[dict[str, int]])
# Revealed type is 'builtins.str'

---

So, are those legal?

Searched but couldn't find docs on it. I do remember seeing @gvanrossum commenting on some issue (couldn't find it now) that mypy is being conservative about allowing user-defined type operators, so I'm a bit afraid of using those.

In pyright it's even crazier, a lot of tricks with tuples are possible with Head[Unpack[Vars]] and Tail[Unpack[Vars]], plus some typevar bounding behaviour that made me notice even higher kinded types implementation is possible using the Lightweight higher-kinded polymorphism encoding, or maybe even a small type-level implementation of lisp

Inizia esaminando gli operatori definiti dall’utente e gli esempi di reveal_type nell’issue, quindi confrontali con mypy/typeshed/stdlib/typing.pyi e con il comportamento di pyright collegato. Determina se le regole di typing documentate affrontano questi costrutti e quale specifica lacuna nella documentazione rimane; l’issue sarà completa solo quando la legalità e l’uso supportato saranno documentati o risolti chiaramente.