Re: -Wshift-negative-value

> > > As I said on 2017-02-08, diagnostics from -Wshift-negative-value are
> > > most probably just noise.
> >
> > Why not disable them then?
>
> Feel free to do so. Good idea.

Actually, this is is a very (very) bad idea. Applying a (left) shift to a
negative value (or shifting by a negative value) is undefined behavior
since (AFAIK) C99 (which is what we - at least in September - wanted to
target).

Quote from (C11) standard (draft) [1]:

"The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits
are filled with zeros. [..] If E1 has a signed type and nonnegative value
[..] otherwise, the behavior is undefined."
N1570 §6.5.7/4

Thus every single one of those warnings is a possible Heisenbug.
Any random optimizer pass of any compiler could horribly break our code. We
could force compilation without any optimization to reduce the risk of
hitting such a case, but that's not really reasonable.

[1]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

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