map with lambda is broken in Python 2; gives '<Erased>' object is not iterable

[gvanrossum]

@Michael0x2a
This is a regression caused by your PR: f61c2ba Overhaul overload semantics, remove erasure, add union math (#5084).

Example (only works with Python 2 -- it's a syntax error with Python 3):

a = [(1, ''), (2, '')]
b = map(lambda (i, s): str(i) + s, a)

Before that PR, this would pass; now it gives

_.py:2: error: '<Erased>' object is not iterable

Simplifying this further by using a list of ints and a lambda with a non-unpacking argument makes the error going away.

Sorry for being so aggressive with priority etc., this prevents us from using master on our internal code base -- we get a dozen or so of errors due to this. It's not limited to map() -- I see it too for reduce(), imap(), and ifilter(), always with lambda (x, y): ....

[gvanrossum]

Actually I'm not sure if it occurs with ifilter() -- the one occurrence is inside an imap() call:

        owners = imap(lambda (k, v): 'owner=' + k,
                ifilter(lambda (k, v): v > SET_SIZE_LOW_CUTOFF, count_by_owner.iteritems()))

All the other functions involved are overloaded. Oh, I also saw an occurrence with max(), which is also overloaded.

[gvanrossum]

There's also one occurence of

'<nothing>' object is not iterable

on a map(None, ...) call -- not a lambda in sight.

Also, it's not due to the typeshed changes -- I didn't bother changing typeshed when bisecting.