false positive "name not defined" with list comprehension in function arguments default value

• Are you reporting a bug, or opening a feature request?

This is a bug I believe

• Please insert below the code you are checking with mypy

def a():
  pass

def b(
    bb=[list_comprehension_argument for list_comprehension_argument in "abc" 
          if list_comprehension_argument]
  ):
  pass

• What is the actual behavior/output?

mypy repro.py
repro.py:5: error: Name 'list_comprehension_argument' is not defined
repro.py:6: error: Name 'list_comprehension_argument' is not defined
Found 2 errors in 1 file (checked 1 source file)

• What is the behavior/output you expect?

I believe there should be no error reported.

• What are the versions of mypy and Python you are using?
  Do you see the same issue after installing mypy from Git master?

this is on mypy from git master, mypy 0.790+dev.2b704df25ca299e6936927be8d41d2ab945cee2a

• What are the mypy flags you are using? (For example --strict-optional)

This is using the default flags.

This only seem to happens if there's another function like this one, before the function with list comprehension in arguments:

def a():
  pass

When removing this other function or putting it after the one with list comprehension in argument, this false positive is not reported, I mean no error are reported for:

def b(
    bb=[list_comprehension_argument for list_comprehension_argument in "abc" 
          if list_comprehension_argument]
  ):
  pass

def a():
  pass

or

def b(
    bb=[list_comprehension_argument for list_comprehension_argument in "abc" 
          if list_comprehension_argument]
  ):
  pass

Reproduce the report by running mypy repro.py with the shown function-order variants on the referenced git-master version. Trace handling for function argument defaults and list-comprehension scopes, then add a regression test covering both orderings; done means no false-positive name errors and the existing suite passes.