Make it possible to obtain non-type arguments in type analyze plugin hooks

[palle-k]

Feature

It should be possible to obtain value expressions used as generic arguments in an AnalyzeTypeContext
For example, given typing.Annotated[builtins.int, "hello world"], I want to obtain "hello world", which is not a type, but a value.

Pitch

This feature would enable type checking of dependencies in FastAPI. FastAPI dependency injection allows specification of dependencies like this:

def my_dependency() -> SomeType:
    return ...


@app.get("/")
def route_handler(arg: Annotated[SomeType, Depends(my_dependency)]):
    return arg.do_something()

In this situation, MyPy should (with the help of a FastAPI specific plugin) be able to verify that the return type of my_dependency matches the type, which is the first generic argument of Annotated.

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Currently, a plugin could theoretically obtain dependencies using get_type_analyze_hook. However, when inspecting the type arguments (ctx.type.args), any non-type argument to Annotated is not accessible within the scope of this hook.

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Furthermore, this could potentially enable additional inspections relating to Pydantic fields.