Re: [PATCH] c++: fix ICE with is_really_empty_class [PR110106]

[Jason Merrill <jason@redhat.com>]

On 7/20/23 17:58, Marek Polacek wrote:
> On Thu, Jul 20, 2023 at 03:51:32PM -0400, Marek Polacek wrote:
>> On Thu, Jul 20, 2023 at 02:37:07PM -0400, Jason Merrill wrote:
>>> On 7/20/23 14:13, Marek Polacek wrote:
>>>> On Wed, Jul 19, 2023 at 10:11:27AM -0400, Patrick Palka wrote:
>>>>> On Tue, 18 Jul 2023, Marek Polacek via Gcc-patches wrote:
>>>>>
>>>>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk and branches?
>>>>>
>>>>> Looks reasonable to me.
>>>>
>>>> Thanks.
>>>>> Though I wonder if we could also fix this by not checking potentiality
>>>>> at all in this case?  The problematic call to is_rvalue_constant_expression
>>>>> happens from cp_parser_constant_expression with 'allow_non_constant' != 0
>>>>> and with 'non_constant_p' being a dummy out argument that comes from
>>>>> cp_parser_functional_cast, so the result of is_rvalue_constant_expression
>>>>> is effectively unused in this case, and we should be able to safely elide
>>>>> it when 'allow_non_constant && non_constant_p == nullptr'.
>>>>
>>>> Sounds plausible.  I think my patch could be applied first since it
>>>> removes a tiny bit of code, then I can hopefully remove the flag below,
>>>> then maybe go back and optimize the call to is_rvalue_constant_expression.
>>>> Does that sound sensible?
>>>>
>>>>> Relatedly, ISTM the member cp_parser::non_integral_constant_expression_p
>>>>> is also effectively unused and could be removed?
>>>>
>>>> It looks that way.  Seems it's only used in cp_parser_constant_expression:
>>>> 10806   if (allow_non_constant_p)
>>>> 10807     *non_constant_p = parser->non_integral_constant_expression_p;
>>>> but that could be easily replaced by a local var.  I'd be happy to see if
>>>> we can actually do away with it.  (I wonder why it was introduced and when
>>>> it actually stopped being useful.)
>>>
>>> It was for the C++98 notion of constant-expression, which was more of a
>>> parser-level notion, and has been supplanted by the C++11 version.  I'm
>>> happy to remove it, and therefore remove the is_rvalue_constant_expression
>>> call.
>>
>> Wonderful.  I'll do that next.
> 
> I found a use of parser->non_integral_constant_expression_p:
> finish_id_expression_1 can set it to true which then makes
> a difference in cp_parser_constant_expression in C++98.  In
> cp_parser_constant_expression we set n_i_c_e_p to false, call
> cp_parser_assignment_expression in which finish_id_expression_1
> sets n_i_c_e_p to true, then back in cp_parser_constant_expression
> we skip the cxx11 block, and set *non_constant_p to true.  If I
> remove n_i_c_e_p, we lose that.  This can be seen in init/array60.C.

Sure, we would need to use the C++11 code for C++98 mode, which is 
likely fine but is more uncertain.

It's probably simpler to just ignore n_i_c_e_p for C++11 and up, along 
with Patrick's suggestion of allowing null non_constant_p with true 
allow_non_constant_p.

Jason