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.
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" }
+}
