Bitshifting negatives in Pike is a little sketchy

Hi all,

In Pike 7.8, the following operations happen:

25 >> 1 → 12
-25 >> 1 → -1
25 >> -1 → Bad argument 2 to >>(). Expected int(0..)|object.
-25 >> -1 → Bad argument 2 to >>(). Expected int(0..)|object.

Only the first one of these is handled correctly.
Bitshifting negative integers is a generally sketchy business in many
languages, but most seem to agree that -25 >> 1 is -13. However, in
Pike, right bitshifting any negative integer results in -1 (which is
rather wrong!).

25 >> -1 and -25 >> -1 are a little undecided. In most languages, such
as Python, Java and Perl, they return 0. In C and Ruby, a negative
right shift turns into a left shift, so they return 50 and -50.
In Python and Go, they raise errors. All three approaches are valid,
but in Pike we hit a fairly opaque unknown program error -- this
should change to match the behaviour of <<(), which is to tell you off
about a negative shift count.


All in all, we should:
* make a negative shifted by a positive work correctly
* make a negative shifted by a negative either work sensibly, or tell
you off properly

Thanks,
Douglas