Re: [PATCH] c++: Fix ICE with ill-formed array list-initialization [PR93712]

[Jason Merrill <jason@redhat.com>]

On 2/19/20 7:30 PM, Marek Polacek wrote:
> 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.

Ah, ck_aggr and ck_list probably should have used u.expr like 
ck_identity and ck_ambig....

> 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.

...or, if we're going to enforce this, ck_ambig will need to change as well.

And build_aggr_conv and build_complex_conv will need adjusting (one way 
or another).

Jason