New error code for slices syntax, refs #10266 - #11279

I don't think this is a great solution. Here's my understanding:

• Users want to use slices in Annotated (Annotated[int, 3:4]).
• Nobody wants to use Dict[x:y]
• But mypy implementation details mean the two cases are handled in the same place

So I think we should allow the Annotated variant (people should be able to put whatever they want in Annotated), and keep erroring on Dict[x:y]. A new error code doesn't make sense to me because this is such a highly specific case, and if we fix Annotated it's difficult to imagine a use case for disabling the new error code.

Is it difficult in the code to allow slices in Annotated but not elsewhere?

We need @Zac-HD here 🙂

Do we really need to only support Annotation? Or should this be allowed for any type annotations including custom ones?

Here's my desired behaviour:

from typing import Annotated, Dict
from torchtyping import TensorType

a: Annotated[int, 1:2]             # no error
b: Dict[int, x:y]                  # as for Dict[int, slice(x,y)]
c: Dict[x:y]                       # as for Dict[slice(x,y)]
t: TensorType["batch":..., float]  # no error - important to support third-party experimentation

And those errors, with my desired suggestions, are:

error: Invalid type comment or annotation  [valid-type]
note:  Suggestion: use slice[...] instead of slice(...)  # use slice[...] instead of `:` slice notation

error: "dict" expects 2 type arguments, but 1 given  [type-arg]
error: Invalid type comment or annotation  [valid-type]
note:  Suggestion: use slice[...] instead of slice(...)  # use `x, y` instead of `:` slice notation

As far as I can tell, the original motivation for erroring out on slices was to direct users from Dict[x:y] to Dict[x, y] - and at the time there was no Annotated type or third-party experiments to cause problems for.

In priority order, it's most important for me that a and t should have no error (that would solve this for me!); then IMO b and c should emit valid-type errors (but no big deal since I'd fix them either way); and finally the improved suggestions would be nice to have if they're not too hard to implement.