On Thu, 3 Jun 2021, Patrick Palka wrote:
> Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in the
> first testcase due to a spurious constness mismatch between the type of
> the template argument &f and the substituted type of F (the substituted
> type has a top-level const). Note that this mismatch doesn't occur with
> object pointers because in that case a call to
> perform_qualification_conversions from convert_nontype_argument
> implicitly adds a top-level const to the argument (via a cast) to match.
>
> This however seems to be a manifestation of a more general conformance
> issue -- that we're not dropping top-level cv-quals after substituting
> into the type of an NTTP as per [temp.param]/6 (we only do so at parse
> time in process_template_parm). This patch makes convert_template_argument
> drop top-level cv-quals accordingly.
>
> Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> trunk?
>
> PR c++/100893
>
> gcc/cp/ChangeLog:
>
> * pt.c (convert_template_argument): Strip top-level cv-quals
> on the substituted type of a non-type template parameter.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/template/param4.C: New test.
> * g++.dg/template/param5.C: New test.
> ---
> gcc/cp/pt.c | 7 ++++++-
> gcc/testsuite/g++.dg/template/param4.C | 10 ++++++++++
> gcc/testsuite/g++.dg/template/param5.C | 7 +++++++
> 3 files changed, 23 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/template/param4.C
> create mode 100644 gcc/testsuite/g++.dg/template/param5.C
>
> diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> index 7211bdc5bbc..66cc88a331f 100644
> --- a/gcc/cp/pt.c
> +++ b/gcc/cp/pt.c
> @@ -8494,7 +8494,12 @@ convert_template_argument (tree parm,
> return error_mark_node;
> }
> else
> - t = tsubst (t, args, complain, in_decl);
> + {
> + t = tsubst (t, args, complain, in_decl);
> + /* Ignore top-level qualifiers on the substituted type of this
> + non-type template parameter, as per [temp.param]/6. */
> + t = cv_unqualified (t);
> + }
Err, shortly after posting I realized we could get top-level cv-quals not
only after substitution, but also after deduction of a decltype(auto)
NTTP (as in nontype-auto19.C below), so the call to cv_unqualified would
need to happen after do_auto_deduction too, like so. Testing in
progress.
-- >8 --
Subject: [PATCH] c++: top-level cv-quals on type of NTTP [PR100893]
Here, we're rejecting the specialization of g<T,F> with T=A, F=&f in
param4.C below due to a spurious constness mismatch between the type of
the template argument &f and the substituted type of F (the substituted
type has a top-level const). Note that this mismatch doesn't occur with
object pointers because in that case a call to
perform_qualification_conversions from convert_nontype_argument
implicitly adds a top-level const to the argument (via a cast) to match.
This however seems to be a manifestation of a more general conformance
issue -- that we're not dropping top-level cv-quals after substituting
into the type of an NTTP as per [temp.param]/6 (we only do so at parse
time in process_template_parm). So this patch makes
convert_template_argument drop top-level cv-quals accordingly.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?
PR c++/100893
gcc/cp/ChangeLog:
* pt.c (convert_template_argument): Strip top-level cv-quals
on the substituted type of a non-type template parameter.
gcc/testsuite/ChangeLog:
* g++.dg/template/param4.C: New test.
* g++.dg/template/param5.C: New test.
* g++.dg/cpp1z/nontype-auto19.C: New test.
* g++.dg/cpp2a/concepts-decltype.C: Don't expect that the
deduced type of a decltype(auto) NTTP has top-level cv-quals.
---
gcc/cp/pt.c | 4 ++++
gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C | 8 ++++++++
gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C | 2 +-
gcc/testsuite/g++.dg/template/param4.C | 10 ++++++++++
gcc/testsuite/g++.dg/template/param5.C | 7 +++++++
5 files changed, 30 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
create mode 100644 gcc/testsuite/g++.dg/template/param4.C
create mode 100644 gcc/testsuite/g++.dg/template/param5.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 7211bdc5bbc..3cac073ed50 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -8496,6 +8496,10 @@ convert_template_argument (tree parm,
else
t = tsubst (t, args, complain, in_decl);
+ /* Drop top-level cv-qualifiers on the substituted/deduced type of
+ this non-type template parameter, as per [temp.param]/6. */
+ t = cv_unqualified (t);
+
if (invalid_nontype_parm_type_p (t, complain))
return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
new file mode 100644
index 00000000000..5b292a6a1e6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/nontype-auto19.C
@@ -0,0 +1,8 @@
+// Verify top-level cv-qualifiers are dropped from the deduced
+// type of a non-type-template parameter, as per [temp.param]/6.
+// { dg-do compile { target c++17 } }
+
+constexpr int x = 42;
+template<decltype(auto) V> decltype(V)& f();
+using type = decltype(f<x>());
+using type = int&;
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
index 13733c645f3..b375f74eb8a 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype.C
@@ -61,7 +61,7 @@ constexpr int Z = 10;
static_assert(deduced_as<0, int>);
static_assert(deduced_as<0, int&>); // { dg-error "invalid variable template" }
-static_assert(deduced_as<Z, const int>);
+static_assert(deduced_as<Z, int>);
static_assert(deduced_as<(Z), const int>); // { dg-error "invalid variable
template" }
static_assert(deduced_as<(Z), const int&>);
diff --git a/gcc/testsuite/g++.dg/template/param4.C
b/gcc/testsuite/g++.dg/template/param4.C
new file mode 100644
index 00000000000..8061ff72ba2
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/param4.C
@@ -0,0 +1,10 @@
+// PR c++/100893
+
+template<class T, typename T::type F> void g() { }
+
+struct A { typedef void (*const type)(); };
+void f();
+template void g<A, &f>();
+
+struct B { typedef void (B::*const type)(); void f(); };
+template void g<B, &B::f>();
diff --git a/gcc/testsuite/g++.dg/template/param5.C
b/gcc/testsuite/g++.dg/template/param5.C
new file mode 100644
index 00000000000..b8c92f4c217
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/param5.C
@@ -0,0 +1,7 @@
+// Verify top-level cv-qualifiers are dropped when determining the substituted
+// type of a non-type-template parameter, as per [temp.param]/6.
+// { dg-do compile { target c++11 } }
+
+template<class T, T V> decltype(V)& f();
+using type = decltype(f<const volatile int, 0>());
+using type = int&;
--
2.32.0.rc2
