WIP: we can now make use of __class_getitem__ for non-generic types

[sobolevn]

Now this code typechecks with no problems:

class A:
    def __class_getitem__(cls, a: int) -> type[A]:
        return cls

A[1]

But, this code raises:

class A:
    def __class_getitem__(cls, a: str) -> type[A]:
        return cls

A[1]  # E: Argument 1 to "__class_getitem__" of "A" has incompatible type "int"; expected "str"

This is a WIP, because I want to know how many things will break because of this. I'm intersted in both primer and test outputs. I will add tests in next commits.

Closes #11501

[sobolevn]

Oh wow, only 4 tests fail! 🎉

 =========================== short test summary info ============================
FAILED mypy/test/testcheck.py::TypeCheckSuite::testMetaclassGetitem
FAILED mypy/test/testcheck.py::TypeCheckSuite::testInvalidTypeApplicationType
FAILED mypy/test/testcheck.py::TypeCheckSuite::testTypeApplicationCrash
FAILED mypy/test/testcheck.py::TypeCheckSuite::testCallableParsingFromExpr

I can totally fix it! 👍

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

schemathesis (https://github.com/schemathesis/schemathesis.git)
+ src/schemathesis/constants.py: note: In member "from_str" of class "CodeSampleStyle":
+ src/schemathesis/constants.py:69: error: "Type[CodeSampleStyle]" has no attribute "__class_getitem__"
+ src/schemathesis/cli/options.py: note: In member "convert" of class "CSVOption":
+ src/schemathesis/cli/options.py:31: error: "Type[Enum]" has no attribute "__class_getitem__"
- src/schemathesis/models.py: note: In member "from_requests" of class "Response":
- src/schemathesis/models.py:785: error: Unexpected keyword argument "status_code" for "Response"

[sobolevn]

Based on primer output, we have a problem with Enum 🤔
Related line: https://github.com/schemathesis/schemathesis/blob/5a09a03d8558ca5cde1ef32b50290c1ba2949980/src/schemathesis/constants.py#L69

[sobolevn]

More problems:

class A:
    def __class_getitem__(cls, a: int) -> type[A]:
        return cls

class B:
    def __class_getitem__(cls, a: int) -> type[B]:
        return cls

x = A | B
x[2]
# out/ex.py:21: error: Bad number of arguments for type alias, expected: 0, given: 1
# out/ex.py:21: error: Type application is only supported for generic classes
# out/ex.py:21: error: Invalid type: try using Literal[2] instead?

class A:
    def __class_getitem__(cls, a: int) -> type[A]:
        return cls

class B:
    def __class_getitem__(cls, a: int) -> type[B]:
        return cls

x: type[A | B]
x[2]
# out/ex.py:21: error: Value of type "Union[Type[A], Type[B]]" is not indexable

[sobolevn]

I am going to change the scope of this PR: __class_getitem__ is going to be ignored for now (so, all will stay the same as it is now). I will only change how metaclasses with __getitem__ work.

Maybe one day I will return to __class_getitem__.