[C/C++ PATCH] Implement -Wshift-overflow (PR c++/55095) (take 3)

[Joseph Myers]

On Wed, 3 Jun 2015, Marek Polacek wrote:

> > Well, ok then.  This new version incorporates Richard S.'s suggestion,
> > and warns even for 1 << 31 in C99/C11 (also in C90 when -Wshift-overflow
> > is explicitely specified).  For C++, it warns about 1 << 31 by default
> > only in C++11 mode, in C++14 never, otherwise only if -Wshift-overflow
> > is specified.
> > 
> > But the fallout seems to be nonnegligible.  So I think the default should
> > be -Wshift-overflow=1 that doesn't warn about 1 << 31, but still rejects
> > e.g. enum { A = 1 << 31 };.  And -Wshift-overflow=2 would warn even about
> > 1 << 31.  (Perhaps this is exactly what you had in mind, but I'm not sure.)
> 
> Done in the following.  I think this is the best approach.  The documentation
> hopefully makes it clear what's the intended behavior.
> 
> Bootstrapped/regtested on x86_64-linux, ok for trunk?

The C changes are OK.

-- 
Joseph S. Myers
joseph@codesourcery.com