Avoid char[] array in tree_def

[Jason Merrill]

On Sat, Oct 31, 2020 at 6:35 PM Jan Hubicka <hubicka@ucw.cz> wrote:

> > On 10/29/20 1:40 PM, Richard Biener wrote:
> > > On Thu, 29 Oct 2020, Jakub Jelinek wrote:
> > >
> > > > On Thu, Oct 29, 2020 at 05:00:40PM +0100, Jan Hubicka wrote:
> > > > > >
> > > > > > That's ugly and will for sure defeat warning / access code
> > > > > > when we access this as char[], no?  I mean, we could
> > > > > > as well use 'int str[1];' here?
> > > > >
> > > > > Well, we always get char pointer via macro that is IMO OK, but I
> am also
> > > > > not very much in love with this.
> > > >
> > > > Do we treat signed char [...]; as typeless storage too, or just
> > > > what the C++ standard requires (i.e. char, unsigned char and
> std::byte
> > > > where the last one is enum type with unsigned char underlying type)?
> > >
> > > All that is covered by is_byte_access_type which includes all
> > > character types (including char16_t and wchar it seems) and std::byte.
> >
> > Well, that's a bug, apparently from the PR94923 patch (r11-499);
> previously
> > it was char, signed char, and unsigned char.  And even that is too much;
> > even C++98 said just char and unsigned char could be used for bytewise
> > access.
> >
> > When C++17 clarified this with the notion of "provides storage", it
> applied
> > to only unsigned char and std::byte, not even the full set of byte-access
> > types.  We still need to allow bytewise access using plain char, but
> perhaps
> > we don't need to treat plain char arrays as typeless.
> >
> > Attributes to say that a particular array does or does not provide
> storage
> > for objects of other types do sound useful.
>
> Thanks a lot for explanation!
> I am adding Martin Sebor to CC. Perhaps you can fix the problem with
> std::byte, and signed char to imply typeless storage and ideally also
> add an attribute? This seem too deep into C++ FE details for me.
>

I've fixed is_byte_access_type; now we shouldn't treat arrays of signed
char as typeless storage.  I don't have time to add the attribute in stage
1.

Jason