On Fri, Feb 14, 2020 at 09:12:58AM +0100, Jason Merrill wrote:
> On 2/13/20 8:56 PM, Marek Polacek wrote:
> > My P0388R4 patch changed build_array_conv to create an identity
> > conversion at the start of the conversion chain.
>
> Hmm, an identity conversion of {} suggests that it has a type, which it
> doesn't in the language. I'm not strongly against it, but what was the
> reason for this change?
There are two reasons:
1) without it we couldn't get to the original expression at the start
of the conversion chain (saved in .u.expr), this is needed in compare_ics:
10660 tree n1 = nelts_initialized_by_list_init (t1);
10661 tree n2 = nelts_initialized_by_list_init (t2);
and nelts_initialized_by_list_init uses conv_get_original_expr for
arrays that have no dimensions.
2) struct conversion says
/* An implicit conversion sequence, in the sense of [over.best.ics].
The first conversion to be performed is at the end of the chain.
That conversion is always a cr_identity conversion. */
and we were breaking that promise.
> > That was a sound change but now we crash in convert_like_real
> >
> > 7457 case ck_identity:
> > 7458 if (BRACE_ENCLOSED_INITIALIZER_P (expr))
> > 7459 {
> > 7460 int nelts = CONSTRUCTOR_NELTS (expr);
> > 7461 if (nelts == 0)
> > 7462 expr = build_value_init (totype, complain);
> > 7463 else if (nelts == 1)
> > 7464 expr = CONSTRUCTOR_ELT (expr, 0)->value;
> > 7465 else
> > 7466 gcc_unreachable (); // HERE
> > 7467 }
>
> Right, this is assuming that any other {} will either be ill-formed or
> handled by ck_aggr or ck_list. How are we getting here without going
> through one of those?
So here we have a ck_aggr which is bad_p, so in convert_like_real we go to the
7285 if (convs->bad_p
block at the beginning, which will look at the next conversion:
7319 for (; t ; t = next_conversion (t))
that conversion is a ck_identity which is not bad_p, so we call
7343 else if (t->kind == ck_user || !t->bad_p)
7344 {
7345 expr = convert_like_real (t, expr, fn, argnum,
7346
/*issue_conversion_warnings=*/false,
7347 /*c_cast_p=*/false,
7348 complain);
and crash there.
Marek
