The behaviour here is just because we don't support type[C[int]] in user annotations yet -- you're hitting this TODO branch: https://github.com/astral-sh/ruff/blob/f44598dc113a7380ca52d6c969431ca929a93473/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs#L683-L686

If you workaround the missing piece in our type-expression parsing, you'll find that we emit the error you expect here:

from ty_extensions import TypeOf

class C[T]:
    pass

class D[T]:
    pass

def f(x: C[int]):
    meta_x = type(x)
    reveal_type(meta_x)  # revealed: type[C[int]]
    _: TypeOf[meta_x] = D[int]  # error[invalid-assignment] "Object of type `<class 'D[int]'>` is not assignable to `type[C[int]]`"

https://play.ty.dev/eb85e9ee-1ce4-420e-bb61-6bfdf6297985

Possibly fixing the TODO branch in https://github.com/astral-sh/ruff/blob/f44598dc113a7380ca52d6c969431ca929a93473/crates/ty_python_semantic/src/types/infer/builder/type_expression.rs#L683-L686 should also be a Beta issue, but it's a pretty different issue to the one originally reported here 😄

support type[...] where ... is a generic alias #1101

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support `type[...]` where `...` is a generic alias #1101