Re: [PATCH] c++: requires-expr substitution and access checking [PR107179]

[Jason Merrill <jason@redhat.com>]

On 11/3/22 11:45, Patrick Palka wrote:
> Like during satisfaction, we need to check access immediately during
> substitution of a requires-expr since the outcome of an access check can
> determine the value of the requires-expr.  And otherwise, in contexts
> where access checking is deferred (such as during substitution into a
> base-clause), a failed access check may leak out from the requires-expr
> into a non-SFINAE context and cause a hard error (as in the testcase
> below).
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?

OK.

> 	PR c++/107179
> 
> gcc/cp/ChangeLog:
> 
> 	* constraint.cc (tsubst_requires_expr): Make sure we're not
> 	deferring access checks.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-requires31.C: New test.
> ---
>   gcc/cp/constraint.cc                             |  3 +++
>   gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C | 13 +++++++++++++
>   2 files changed, 16 insertions(+)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C
> 
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index 5e6a3bcf059..f6ef078171a 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -2252,6 +2252,9 @@ tsubst_requires_expr (tree t, tree args, sat_info info)
>   {
>     local_specialization_stack stack (lss_copy);
>   
> +  /* We need to check access during the substitution.  */
> +  deferring_access_check_sentinel acs (dk_no_deferred);
> +
>     /* A requires-expression is an unevaluated context.  */
>     cp_unevaluated u;
>   
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C b/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C
> new file mode 100644
> index 00000000000..9b7e2a34889
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-requires31.C
> @@ -0,0 +1,13 @@
> +// PR c++/107179
> +// { dg-do compile { target c++20 } }
> +
> +template<bool B> struct bool_constant { static constexpr bool value = B; };
> +
> +template<typename T>
> +  struct is_implicitly_default_constructible
> +  : bool_constant<requires { T(); }>
> +  { };
> +
> +struct X { private: X(); };
> +
> +static_assert( !is_implicitly_default_constructible<X>::value );