Re: [PATCH] c++: explicit spec of constrained member tmpl [PR107522]

[Jason Merrill <jason@redhat.com>]

On 12/1/22 11:37, Patrick Palka wrote:
> When defining a explicit specialization of a constrained member template
> (of a class template) such as f and g in the below testcase, the
> DECL_TEMPLATE_PARMS of the corresponding TEMPLATE_DECL are partially
> instantiated, whereas its associated constraints are carried over
> from the original template and thus are in terms of the original
> DECL_TEMPLATE_PARMS.

But why are they carried over?  We wrote a specification of the 
constraints in terms of the temprate parameters of the specialization, 
why are we throwing that away?

> So during normalization for such an explicit
> specialization we need to consider the (parameters of) the most general
> template, since that's what the constraints are in terms of and since we
> always use the full set of template arguments during satisfaction.
> 
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk and perhaps 12?
> 
> 	PR c++/107522
> 
> gcc/cp/ChangeLog:
> 
> 	* constraint.cc (get_normalized_constraints_from_decl): Use the
> 	most general template for an explicit specialization of a
> 	member template.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/cpp2a/concepts-explicit-spec7.C: New test.
> ---
>   gcc/cp/constraint.cc                          | 18 ++++++++---
>   .../g++.dg/cpp2a/concepts-explicit-spec7.C    | 31 +++++++++++++++++++
>   2 files changed, 44 insertions(+), 5 deletions(-)
>   create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
> 
> diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
> index ab0f66b3d7e..f1df84c2a1c 100644
> --- a/gcc/cp/constraint.cc
> +++ b/gcc/cp/constraint.cc
> @@ -973,11 +973,19 @@ get_normalized_constraints_from_decl (tree d, bool diag = false)
>        accepting the latter causes the template parameter level of U
>        to be reduced in a way that makes it overly difficult substitute
>        concrete arguments (i.e., eventually {int, int} during satisfaction.  */
> -  if (tmpl)
> -  {
> -    if (DECL_LANG_SPECIFIC(tmpl) && !DECL_TEMPLATE_SPECIALIZATION (tmpl))
> -      tmpl = most_general_template (tmpl);
> -  }
> +  if (tmpl && DECL_LANG_SPECIFIC (tmpl)
> +      && (!DECL_TEMPLATE_SPECIALIZATION (tmpl)
> +	  /* DECL_TEMPLATE_SPECIALIZATION means we're dealing with either a
> +	     partial specialization or an explicit specialization of a member
> +	     template.  In the former case all is well: the constraints are in
> +	     terms in TMPL's parameters.  But in the latter case TMPL's
> +	     parameters are partially instantiated whereas its constraints
> +	     aren't, so we need to consider (the parameters of) the most
> +	     general template.  The following test distinguishes between a
> +	     partial specialization and such an explicit specialization.  */
> +	  || (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (tmpl))
> +	      < TMPL_ARGS_DEPTH (DECL_TI_ARGS (tmpl)))))
> +    tmpl = most_general_template (tmpl);
>   
>     d = tmpl ? tmpl : decl;
>   
> diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
> new file mode 100644
> index 00000000000..5b5a6df20ff
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
> @@ -0,0 +1,31 @@
> +// PR c++/107522
> +// { dg-do compile { target c++20 } }
> +
> +template<class T>
> +struct A
> +{
> +  template<int N>
> +  static void f() requires (N == 42);
> +
> +  template<class U>
> +  struct B {
> +    template<int N>
> +    static void g() requires (T(N) == 42);
> +  };
> +};
> +
> +template<>
> +template<int N>
> +void A<int>::f() requires (N == 42) { }
> +
> +template<>
> +template<>
> +template<int N>
> +void A<int>::B<int>::g() requires (int(N) == 42) { }
> +
> +int main() {
> +  A<int>::f<42>();
> +  A<int>::f<43>(); // { dg-error "no match" }
> +  A<int>::B<int>::g<42>();
> +  A<int>::B<int>::g<43>(); // { dg-error "no match" }
> +}