MyPy should report error if variable imported in TYPE_CHECKING block?

Currently, MyPy doesn't report an error when a variable is imported in the if TYPE_CHECKING block and then used elsewhere.
For example, MyPy accepts this code:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from typing import NamedTuple

class C(NamedTuple):
    x: int
    y: int

even though, at runtime, it fails:

Traceback (most recent call last):
  File "type_checking_test.py", line 6, in <module>
    class C(NamedTuple):
NameError: name 'NamedTuple' is not defined

(I'm using MyPy version 0.650.)

Should this be fixed? I think this could be fixed by adding an is_type_checking_only variable to SymbolTableNode, which is True if the variable was imported or assigned within a TYPE_CHECKING block, and ensuring that this flag is false for any variables used in places other than type annotations in the actual code.

Start by reproducing the issue with the provided TYPE_CHECKING and NamedTuple example, then trace how imports and symbol-table entries from TYPE_CHECKING blocks are handled. The work is done when MyPy reports the runtime-invalid use while continuing to allow names used only in type annotations, with coverage for the example behavior.