Typechecker does not check that "types" are actually types

[sid-kap]

MyPy does not show an error for the following code:

import typing

def foo(s: set[int]) -> int:
  return len(s)

def goo(l: list[int]) -> int:
  return len(l)

However, this code is not actually valid: it throws the following error at runtime:

Traceback (most recent call last):
  File "error.py", line 3, in <module>
    def foo(s: set[int]) -> int:
TypeError: 'type' object is not subscriptable

The first step to fixing this code is to change set to Set and list to List. This, however, leads to another runtime error:

Traceback (most recent call last):
  File "error.py", line 3, in <module>
    def foo(s: Set[int]) -> int:
NameError: name 'Set' is not defined

Finally, we need to add the following imports:

from typing import Set, List

and the program finally runs without an error.

So there are two issues here:

1. mypy should catch when a "not subscriptable" thing (whatever that means) is written instead of an actual type
2. mypy should detect when types are used without being imported

[JukkaL]

Yeah, these are (known) bugs. I'm not sure if there is another issue created for this.

I believe that this only affects List, Dict and Set because they are special cased as aliases to list, dict and set, respectively. Mypy should not consider List and list interchangeable, as it currently does. However, list should still remain a valid type and be equivalent to List[Any].

[gvanrossum]

This is probably a dupe of #999.