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

[Marek Polacek <polacek@redhat.com>]

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.)

Thanks,

> > 
> > -- >8 --
> > 
> > is_really_empty_class is liable to crash when it gets an incomplete
> > or dependent type.  Since r11-557, we pass the yet-uninstantiated
> > class type S<0> of the PARM_DECL s to is_really_empty_class -- because
> > of the potential_rvalue_constant_expression -> is_rvalue_constant_expression
> > change in cp_parser_constant_expression.  Here we're not parsing
> > a template so we did not check COMPLETE_TYPE_P as we should.
> > 
> > 	PR c++/110106
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constexpr.cc (potential_constant_expression_1): Check COMPLETE_TYPE_P
> > 	even when !processing_template_decl.
> > 
> > gcc/testsuite/ChangeLog:
> > 
> > 	* g++.dg/cpp0x/noexcept80.C: New test.
> > ---
> >  gcc/cp/constexpr.cc                     |  2 +-
> >  gcc/testsuite/g++.dg/cpp0x/noexcept80.C | 12 ++++++++++++
> >  2 files changed, 13 insertions(+), 1 deletion(-)
> >  create mode 100644 gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> > 
> > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > index 6e8f1c2b61e..1f59c5472fb 100644
> > --- a/gcc/cp/constexpr.cc
> > +++ b/gcc/cp/constexpr.cc
> > @@ -9116,7 +9116,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
> >        if (now && want_rval)
> >  	{
> >  	  tree type = TREE_TYPE (t);
> > -	  if ((processing_template_decl && !COMPLETE_TYPE_P (type))
> > +	  if (!COMPLETE_TYPE_P (type)
> >  	      || dependent_type_p (type)
> >  	      || is_really_empty_class (type, /*ignore_vptr*/false))
> >  	    /* An empty class has no data to read.  */
> > diff --git a/gcc/testsuite/g++.dg/cpp0x/noexcept80.C b/gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> > new file mode 100644
> > index 00000000000..3e90af747e2
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp0x/noexcept80.C
> > @@ -0,0 +1,12 @@
> > +// PR c++/110106
> > +// { dg-do compile { target c++11 } }
> > +
> > +template<int> struct S
> > +{
> > +};
> > +
> > +struct G {
> > +  G(S<0>);
> > +};
> > +
> > +void y(S<0> s) noexcept(noexcept(G{s}));
> > 
> > base-commit: fca089e8a47314a40ad93527ba9f9d0d374b3afb
> > -- 
> > 2.41.0
> > 
> > 
> 

Marek