Unexpected behavior on using NamedTuple and Generic when bounding TypeVar in different ways

[yoyoLeeTW]

Crash Report / Traceback / To Reproduce

I try to use NamedTuple and Generic such as the following code:

# test.py
from typing import (NamedTuple, Generic, TypeVar)

S = TypeVar('S', bound='_S')

class _S:
    pass

class A(NamedTuple, Generic[S]):
    s: S

In this case mypy raises an internal error:

test.py:9: error: INTERNAL ERROR -- Please try using mypy master on Github:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 0.902
Traceback (most recent call last):
  File "mypy/semanal.py", line 4872, in accept
  File "mypy/nodes.py", line 687, in accept
  File "mypy/semanal.py", line 577, in visit_func_def
  File "mypy/semanal.py", line 609, in analyze_func_def
  File "mypy/semanal.py", line 4583, in defer
AssertionError: Must not defer during final iteration
test.py:9: : note: use --pdb to drop into pdb

But mypy returns "sucess" with code 0 if I swap class _S and S:

# test.py
from typing import (NamedTuple, Generic, TypeVar)

class _S:
    pass

S = TypeVar('S', bound='_S')

class A(NamedTuple, Generic[S]):
    s: S

Your Environment

• Mypy version used: 0.902
• Mypy command-line flags: mypy --show-traceback test.py
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.0
• Operating system and version: Ubuntu 18.04.5

[AlexWaygood]

Same assertion is hit in #12113, #10140, #11728, #12252, #10672 and #12112

[ilevkivskyi]

Both examples now pass on master without errors