On Thu, 1 Dec 2022, Jason Merrill wrote:
> 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?
Using the partially instantiated constraints would require adding a
special case to satisfaction since during satisfaction we currently
always use the full set of template arguments (relative to the most
general template). For satisfaction of the partiall instantiated
constraints, we'd instead have to use the template arguments relative to
the explicit specialization, e.g. {42} instead of {{int},{42}} for
A<int>::f<42>. Not sure if that would be preferable, but it seems
doable.
>
> > 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" }
> > +}
>
>
