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

[Marek Polacek <polacek@redhat.com>]

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.

Marek