Re: [PATCH] c++: partial spec constraint checking context [PR105220]

[Jason Merrill <jason@redhat.com>]

On 5/2/22 14:50, Patrick Palka wrote:
> Currently when checking the constraints of a class template, we do so in
> the context of the template, not the specialized type.  This is the best
> we can do for a primary template since the specialized type is valid
> only if the primary template's constraints are satisfied.

Hmm, that's unfortunate.  It ought to be possible, if awkward, to form 
the type long enough to check its constraints.

> But for a
> partial specialization, we can assume the specialized type is valid (as
> a consequence of constraints being checked only when necessary), so we
> arguably should check the constraints on a partial specialization more
> specifically in the context of the specialized type, not the template.
> 
> This patch implements this by substituting and setting the access
> context appropriately in satisfy_declaration_constraints.  Note that
> setting the access context in this case is somewhat redundant since the
> relevant caller most_specialized_partial_spec will already have set the
> access context to the specialiation, but this redundancy should be harmless.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk and perhaps 12.2 (after the branch is thawed)?
> 
> 	PR c++/105220
> 
> gcc/cp/ChangeLog:
> 
> 	* constraint.cc (satisfy_declaration_constraints): When checking
> 	the constraints of a partial template specialization, do so in
> 	the context of the specialized type not the template.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-partial-spec12.C: New test.
> ---
>   gcc/cp/constraint.cc                          | 17 ++++++++++++++---
>   .../g++.dg/cpp2a/concepts-partial-spec12.C    | 19 +++++++++++++++++++
>   2 files changed, 33 insertions(+), 3 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
> 
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index 94f6222b436..772f8532b47 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -3253,11 +3253,22 @@ satisfy_declaration_constraints (tree t, tree args, sat_info info)
>       {
>         if (!push_tinst_level (t, args))
>   	return result;
> -      tree pattern = DECL_TEMPLATE_RESULT (t);
> +      tree ascope = DECL_TEMPLATE_RESULT (t);
> +      if (CLASS_TYPE_P (TREE_TYPE (t))
> +	  && CLASSTYPE_TEMPLATE_SPECIALIZATION (TREE_TYPE (t)))
> +	{
> +	  gcc_checking_assert (t == most_general_template (t));
> +	  /* When checking the constraints on a partial specialization,
> +	     do so in the context of the specialized type, not the template.
> +	     This substitution should always succeed since we shouldn't
> +	     be checking constraints thereof unless the specialized type
> +	     is valid.  */
> +	  ascope = tsubst (ascope, args, tf_none, info.in_decl);
> +	}
>         push_to_top_level ();
> -      push_access_scope (pattern);
> +      push_access_scope (ascope);
>         result = satisfy_normalized_constraints (norm, args, info);
> -      pop_access_scope (pattern);
> +      pop_access_scope (ascope);
>         pop_from_top_level ();
>         pop_tinst_level ();
>       }
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
> new file mode 100644
> index 00000000000..641d456722d
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-partial-spec12.C
> @@ -0,0 +1,19 @@
> +// PR c++/105220
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +concept fooable = requires(T t) { t.foo(); };
> +
> +template<class>
> +struct A;        // #1, incomplete
> +
> +template<fooable T>
> +struct A<T> { }; // #2
> +
> +struct B {
> +private:
> +  friend struct A<B>;
> +  void foo();
> +};
> +
> +template struct A<B>; // OK, B::foo() is accessible from #2