Undefined comprehension variable in class body not caught

class Test:
    a = [1, 2, 3]
    b = [1, 2, 3]
    c = [i*j for i in a for j in b]

Pyflakes gives no errors here. But believe it or not, this raises a NameError

Traceback (most recent call last):
  File "test.py", line 1, in <module>
    class Test:
  File "test.py", line 4, in Test
    c = [i*j for i in a for j in b]
  File "test.py", line 4, in <listcomp>
    c = [i*j for i in a for j in b]
NameError: name 'b' is not defined

The reason has to do with how Python treats the outermost comprehension iterable differently. See https://stackoverflow.com/questions/13905741/accessing-class-variables-from-a-list-comprehension-in-the-class-definition. Note that the following does not give any errors when executed

class Test:
    a = [1, 2, 3]
    b = [1, 2, 3]
    c = [i for i in a]

No project file or test is named. Start by reproducing the class-body comprehension and tracing pyflakes' comprehension and scope analysis; compare the diagnostic for the undefined inner iterable with the valid single-iterable example. Done means the reported NameError case is flagged without incorrectly flagging the valid case.