global/local syntax errors not caught

Original report by asmeurer (@asmeurer?) on Launchpad:

This is perhaps a bit obscure, but here's a class of syntax errors that aren't caught by pyflakes

$ cat test.py
def test(x):
    global x
$ pyflakes test.py
$ python test.py
  File "test.py", line 2
    global x
SyntaxError: name 'x' is parameter and global

I'm unsure what the full set of possible errors are here. One would need to check the Python source. I know there are some others relating to the Python 3 "nonlocal" keyword as well.

Start by reproducing the test.py example with pyflakes and compare its output with Python's syntax error. Review the Python source for related global and nonlocal errors, then define the complete set that should be reported and add coverage for them. Done means these syntax errors are detected consistently.