Re: [PATCH] PR libstdc++/80624 satisfy invariant for char_traits<char16_t>::eof()

["Florian Weimer via gcc-patches" <gcc-patches at gcc dot gnu dot org>]

On 05/08/2017 12:24 PM, Jonathan Wakely wrote:
> On 08/05/17 11:53 +0200, Florian Weimer via libstdc++ wrote:
> > On 05/05/2017 07:05 PM, Jonathan Wakely wrote:
> > > As discussed at http://stackoverflow.com/q/43769773/981959 (and kinda
> > > hinted at by http://wg21.link/lwg1200) there's a problem with
> > > char_traits<char16_t>::eof() because it returns int_type(-1) which is
> > > the same value as u'\uFFFF', a valid UTF-16 code point.
> >
> > I think the real bug is that char_traits<char16_t>::int_type is just 
> > plain wrong.  It has to be a signed integer,
>
> Why does it have to be signed?

Hmm.  Maybe it's not strictly required.  int_type(-1) as a distinct 
value is likely sufficient.

> > and capable of representing values in the range 0 .. 65535.  
> > char_traits<char32_t> has a similar problem.  char_traits<wchar_t> 
> > should be fine on glibc because WEOF is reserved, something that is 
> > probably not the case for char32_t.
>
> I think there are 32-bit values which are not valid UTF-32 code
> points, including char32_t(-1) which we use for EOF.

I'm not sure if char32_t is restricted to UTF-32 codepoints (the 
standard does not say, I think).  But even UCS-4 is 31-bit only, so 
maybe the problem does not arise there.

Thanks,
Florian