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

[Patrick Palka <ppalka@redhat.com>]

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.

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'.

Relatedly, ISTM the member cp_parser::non_integral_constant_expression_p
is also effectively unused and could be removed?

> 
> -- >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
> 
>