(Sorry to resurrect this thread so late, I lost track of this patch...)
On Fri, 2 Dec 2022, Jason Merrill wrote:
> On 12/2/22 09:30, Patrick Palka wrote:
> > On Thu, 1 Dec 2022, Jason Merrill wrote:
> >
> > > On 12/1/22 14:51, Patrick Palka wrote:
> > > > 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 template 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).
> > >
> > > But not for partial specializations, right? It seems natural to handle
> > > this
> > > explicit instantiation the way we handle partial specializations, as both
> > > have
> > > their constraints written in terms of their template parameters.
> >
> > True, but what about the general rule that we don't partially instantiate
> > constraints outside of declaration matching? Checking satisfaction of
> > partially instantiated constraints here can introduce hard errors during
> > normalization, e.g.
> >
> > template<class T>
> > concept C1 = __same_as(T, void);
> >
> > template<class T>
> > concept C2 = C1<typename T::type>;
> >
> > template<int N>
> > concept D = (N == 42);
> >
> > template<class T>
> > struct A {
> > template<int N>
> > static void f() requires C2<T> || D<N>;
> > };
> >
> > template<>
> > template<int N>
> > void A<int>::f() requires C2<int> || D<N> { }
> >
> > int main() {
> > A<int>::f<42>();
> > }
> >
> > Normalization of the the partially instantiated constraints will give a
> > hard error due to 'int::type' being ill-formed, whereas the uninstantiated
> > constraints are fine.
>
> Hmm, interesting point, but in this example that happens because the
> specialization is nonsensical: we wouldn't be normalizing the
> partially-instantiated constraints so much as the ones that the user
> explicitly wrote, so a hard error seems justified.
While the written partially-instantiated constraints are nonsensical,
aren't they only needed for sake of declaration matching? It doesn't
seem to necessarily imply that that form of constraints is what should
prevail. This is where the analogy with partial specializations breaks
down IMHO: partial specializations own their constraints.
Implementing your desired approach isn't so bad either however. We
mainly just need to correct for TI_ARGS being relative to the primary
template rather than the partially instantiated template. Something
like the following?
-- >8 --
Subject: [PATCH] c++: explicit spec of constrained member tmpl [PR107522]
PR c++/107522
gcc/cp/ChangeLog:
* constraint.cc (satisfy_declaration_constraints): Remove
extraneous outer arguments for a partial or explicit
specialization.
* pt.cc (determine_specialization): For an explicit
specialization of a member template, make the partially
instantiated constraints prevail.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-explicit-spec7.C: New test.
---
gcc/cp/constraint.cc | 14 ++++++++-
gcc/cp/pt.cc | 7 ++++-
.../g++.dg/cpp2a/concepts-explicit-spec7.C | 30 +++++++++++++++++++
3 files changed, 49 insertions(+), 2 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 ebfcdefd284..4dc4fedc659 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2728,6 +2728,11 @@ satisfy_declaration_constraints (tree t, sat_info info)
args = TI_ARGS (ti);
if (inh_ctor_targs)
args = add_outermost_template_args (args, inh_ctor_targs);
+ if (DECL_TEMPLATE_SPECIALIZATION (TI_TEMPLATE (ti)))
+ {
+ tree parms = DECL_TEMPLATE_PARMS (TI_TEMPLATE (ti));
+ args = get_innermost_template_args (args, TMPL_PARMS_DEPTH (parms));
+ }
}
if (regenerated_lambda_fn_p (t))
@@ -2811,7 +2816,14 @@ satisfy_declaration_constraints (tree t, tree args,
sat_info info)
args = add_to_template_args (outer_args, args);
}
else
- args = add_outermost_template_args (t, args);
+ {
+ args = add_outermost_template_args (t, args);
+ if (DECL_TEMPLATE_SPECIALIZATION (t))
+ {
+ tree parms = DECL_TEMPLATE_PARMS (t);
+ args = get_innermost_template_args (args, TMPL_PARMS_DEPTH (parms));
+ }
+ }
/* If the innermost arguments are dependent, or if the outer arguments
are dependent and are needed by the constraints, we can't check
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 310e5dfff03..04987f66746 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -2502,7 +2502,12 @@ determine_specialization (tree template_id,
*targs_out = copy_node (DECL_TI_ARGS (fn));
/* Propagate the candidate's constraints to the declaration. */
- if (tsk != tsk_template)
+ if (tsk == tsk_template)
+ {
+ remove_constraints (fn);
+ set_constraints (fn, get_constraints (decl));
+ }
+ else
set_constraints (decl, get_constraints (fn));
/* DECL is a re-declaration or partial instantiation of a template
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..9452159faf7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-explicit-spec7.C
@@ -0,0 +1,30 @@
+// 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" }
+}
--
2.46.0.551.gc5ee8f2d1c
>
> > > > For satisfaction of the partially 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" }
> > > > > > +}
> > > > >
> > > > >
> > > >
> > >
> > >
> >
>
>
