Re: [PR c++/78572] handle array self references in intializers

[Jakub Jelinek <jakub at redhat dot com>]

On Wed, Dec 21, 2016 at 12:29:37PM -0500, Aldy Hernandez wrote:
> > > int array[10] = { array[3]=5, 0x111, 0x222, 0x333 };
> > > 
> > > (gdb) x/4x &array
> > > 0x601040 <array>:       0x00000005      0x00000111      0x00000222
> > > 0x00000005
> > > 
> > > That is, the array[3]=5 overwrites the last 0x333.  I would expect
> > > that...
> > 
> > That may be wrong. Using the 4431 draft, [8.5.4]/4 says 'Within the
> > initializer-list of a braced-init-list, the initializer-clauses,
> > including any that result from pack expansions (14.5.3), are evaluated
> > in the order in which they appear.'
> > It goes on to mention that there are sequence points, plus that the
> > ordering holds regardless of the semantics of initialization.
> > 
> > So by my reading, the effects of the above list are:
> > a) assign 5 to array[3]
> > b) assign 5 to array[0] -- consume the first initializer
> > c) assign x111 to array[1] -- consume the second initializer
> > d) assign 0x222 to array[2] -- consume the third initializer
> > e) assign 0x333 to array[3] -- consume the fourth intializer,
> >                                overwrite #a
> 
> Hmmmm, I see.
> 
> Since designated initializers in arrays are not supported in the C++ FE,
> should we sorry() on array initializers using self-reference like above the
> above?  Surely it's better than ICEing.  Or, would you prefer we implement
> the above semantics in the draft for GCC7?

The side-effects are unrelated to designed initializers, only [3] = 5
is designed initializer, array[3] = 5 is not, it is just arbitrary
expression evaluated at that point to compute the corresponding element.
We do support designed initializers in C++, but only in a useless way
(require that the designators appear in order without any gaps).

	Jakub