NamedTuple with dependent default values causes unexpected 'Name is not defined' errors

Hi there,

When there is a NamedTuple with a default value that depends on one or more other default values, followed by a default value that should resolve into a NameError, all preceding default values get the Name <variable> is not defined error as well.

E.g.

from typing import NamedTuple

class Example(NamedTuple):
    a: int = 1
    b: int = 2
    c: int = a*b
    d: int = does_not_exist*5

Causes:

scratch.py:6: error: Name 'a' is not defined
scratch.py:6: error: Name 'b' is not defined
scratch.py:7: error: Name 'does_not_exist' is not defined

If I comment out line 6, I do, however, get the expected result.

from typing import NamedTuple

class Example(NamedTuple):
    a: int = 1
    b: int = 2
    # c: int = a*b
    d: int = does_not_exist*5

scratch.py:7: error: Name 'does_not_exist' is not defined

This was tried on version 0.760.

$ mypy --version
mypy 0.760

Start by reproducing the issue from the scratch.py NamedTuple example using mypy 0.760, then inspect the NamedTuple type-checking path responsible for default values. Compare the dependent-default case with the version where c is commented out. Done means the undefined name in d is reported without spurious errors for a, b, or c.