When instantiating a constrained hidden template friend, we need to
substitute into its constraints for sake of declaration matching. For
this substitution we use a full argument vector whose outer levels
correspond to the class's arguments and innermost level corresponds to
the template's level-lowered generic arguments.
But for A<int>::f here, for which the relevant argument vector is
{{int}, {Us...}}, this substitution triggers the assert in
use_pack_expansion_extra_args_p since one argument is a pack expansion
and the other isn't.
And for A<int, int>::f, for which the relevant argument vector is
{{int, int}, {Us...}}, the use_pack_expansion_extra_args_p assert would
also trigger but we first get a bogus "mismatched argument pack lengths"
error from tsubst_pack_expansion.
These might ultimately be bugs in tsubst_pack_expansion, but it seems
we can work around them by tweaking the constraint substitution in
maybe_substitute_reqs_for to only use the friend's outer arguments in
the case of a template friend. This should be otherwise equivalent to
substituting using the full arguments, since the template's innermost
arguments are just its generic arguments with level=1.
Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk/12?
PR c++/108066
PR c++/108067
gcc/cp/ChangeLog:
* constraint.cc (maybe_substitute_reqs_for): For a template
friend, substitute using only its outer arguments. Remove
dead uses_template_parms test.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-friend12.C: New test.
---
gcc/cp/constraint.cc | 8 +++++++
.../g++.dg/cpp2a/concepts-friend12.C | 22 +++++++++++++++++++
2 files changed, 30 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index d4cd92ec3b4..f9d1009c9fe 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1352,6 +1352,14 @@ maybe_substitute_reqs_for (tree reqs, const_tree decl)
tree tmpl = DECL_TI_TEMPLATE (decl);
tree gargs = generic_targs_for (tmpl);
processing_template_decl_sentinel s;
+ if (PRIMARY_TEMPLATE_P (tmpl))
+ {
+ if (TEMPLATE_ARGS_DEPTH (gargs) == 1)
+ return reqs;
+ ++processing_template_decl;
+ gargs = copy_node (gargs);
+ --TREE_VEC_LENGTH (gargs);
+ }
if (uses_template_parms (gargs))
++processing_template_decl;
reqs = tsubst_constraint (reqs, gargs,
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
new file mode 100644
index 00000000000..95973842afb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
@@ -0,0 +1,22 @@
+// PR c++/108066
+// PR c++/108067
+// { dg-do compile { target c++20 } }
+
+template<class T, class U>
+concept C = __is_same(T, U);
+
+template<class... Ts>
+struct A {
+ template<class... Us>
+ requires (... && C<Ts, Us>)
+ friend void f(A, A<Us...>) { }
+};
+
+int main() {
+ A<int> x;
+ f(x, x);
+ A<int, int> y;
+ f(y, y);
+ A<char> z;
+ f(x, z); // { dg-error "no match" }
+}
--
2.39.0.rc2.23.g694cb1b2ab
