Cannot instantiate `Foo` because it is a protocol [bad-instantiation]

[MarcoGorelli]

Describe the Bug

The following case satisfies other type checkers, but pyrefly throws

ERROR Cannot instantiate `Foo` because it is a protocol [bad-instantiation]
 --> t.py:7:26
  |
7 |         return type(self)(3)
  |                          ^^^
  |
 INFO 1 error

from typing import Protocol, Self

class Foo(Protocol):
    def __init__(self, a: int) -> None:
        ...
    def foo(self) -> Self:
        return type(self)(3)

Spotted in Narwhals

Sandbox Link

https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0AFag1wBjXFAA0dAMowoYADrploqKjhw6AMVy4AFENwjxUAJSJlda3UwwwdAPqPWEBs-1x5YaakSd0BjM6AFoAPjoAOVx0GEt0G0S6QhSrGzsHMD1Pb2Dw2W94pJtKGAYAV0oEpmIYHIUzfQBmM2VlEEkQcoZoOBJyRBAAYjoAVR6oN1I6MHL0UR6YuDasexneGlR3dHKabBhKfXx-ViDQiLgGSiKSssqEpRBI3f3rumB8AF9FEHbOslKYCgpEIIhoUAoIwEpEBwLoaCweHwdHE6EgbEqWwgMUIyhGchgdAAFgwGMQ4IgAPSUgH2YGEXhsSkwdCUzBiOCU1HozGLVnrSh0VAAN1Q0FQ2FgKJiPMoWJidFwxD5fWUZAYRJiIWFBzg2ISAF46D8moQAIwAJh%2B7U%2BnVQCwgOu00BgFAROAI-RAnyAA

(Only applicable for extension issues) IDE Information

No response

[kinto0]

this seems to me like a bug with pyright. pyright prevents protocols from being instantiated but fails to identify that type(self) is a protocol:
https://pyright-play.net/?code=GYJw9gtgBALgngBwJYDsDmUkQWEMoAK4MYAxmADYA0UAygKYXABQzpFAhgM5dQBiYMAAoiYEuQoBKAFzMo8qABN6wKAH01qJDA1CujYDQ7TMKGJKgBaAHxQAcmBT1ZC11AB0nuQuWrggvQMLGzoDFzd5EHoAN3oOCjV4BHohJJT9JklJbwiomABXEBRYRHSgoQBmbOYOKABefgCqoA

[dangotbanned]

Pyrefly is fine with this Sandbox link

(@MarcoGorelli you might have seen me use this pattern before):

class Foo2(Protocol):
    number: float
    @classmethod
    def from_int(cls, integer: int) -> Self:
        obj = cls.__new__(cls)
        obj.number = float(integer)
        return obj

[kinto0]

closing due to workaround, feel free to reopen if you think the UX could be improved

[MarcoGorelli]

sure, thanks all!

[MarcoGorelli]

@dangotbanned is there any issue with just using

    def foo(self) -> Self:
        return self.__class__(3)

here?

looks like pyrefly's ok with that? https://pyrefly.org/sandbox/?project=N4IgZglgNgpgziAXKOBDAdgEwEYHsAeAdAA4CeS4ATrgLYAEALqcROgOZ0Q3G6UN0AFag1wBjXFAA0dAMowoYADrploqKjhw6AMVy4AFENwjxUAJSJlda3UwwwdAPqPWEBs-1x5YaakSd0BjM6AFoAPjoAOVx0GEt0G0S6QhSrGzsHMD1Pb2Dw2W94pJtKGAYAV0oErwVCZzUNOA8AZjNlZRBJEHKGaDgSckQQAGI6AFVeqDdSOjBy9FFemLh2rHtZ3hpUd3RymmwYSn18f1Yg0Ii4BkoikrLKhKUQSL2Dm7pgfABfRRAOrrIpTAUFIhBENCgFFGAlIQJBdDQWDw%2BDo4nQkDYlW2EBihGUozkMDoAAsGAxiHBEAB6KmA%2Bwgwi8NhUmDoKmYMRwKlojFYpZsjaUOioABuqGgqGwsFRMV5lGxMTouGI-P6yjIDGJMRCIsOcBxCQAvHRfs1CABGABMvw6Xy6qEWEF12mgMAoiJwBAGIC%2BQA