Spurious "redefinition of unused" for singledispatch implementations

[Antony Lee]

Consider the following example, straight from the official docs for `functools.singledispatch` (https://docs.python.org/3.5/library/functools.html#functools.singledispatch):

from functools import singledispatch
@singledispatch
def fun(arg, verbose=False):
    if verbose:
        print("Let me just say,", end=" ")
    print(arg)
@fun.register(int)
def _(arg, verbose=False):
    if verbose:
        print("Strength in numbers, eh?", end=" ")
    print(arg)
@fun.register(list)
def _(arg, verbose=False):
    if verbose:
        print("Enumerate this:")
    for i, elem in enumerate(arg):
        print(i, elem)

pyflakes 1.5.0 spuriously reports:

foo.py:13: redefinition of unused '_' from line 7

(In practice I would avoid warning on assignments to '_' as much as possible, and possible warn on *uses* of '_' as much as possible -- although it may actually be used by gettext for i18n?)