c++: template friend with variadic constraints [PR107853]

When instantiating a constrained hidden template friend, we substitute
into its template-head requirements in tsubst_friend_function.  For this
substitution we use the template's full argument vector whose outer
levels correspond to the instantiated class's arguments and innermost
level corresponds to the template's own level-lowered generic arguments.

But for A<int>::f here, for which the relevant argument vector is
{{int}, {Us...}}, the substitution into (C<Ts, Us> && ...) 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.

Sidestepping the question of whether tsubst_pack_expansion should be
able to handle such substitutions, it seems we can work around this by
using only the instantiated class's arguments and not also the template
friend's own generic arguments, which is consistent with how we normally
substitute into the signature of a member template.

	PR c++/107853

gcc/cp/ChangeLog:

	* constraint.cc (maybe_substitute_reqs_for): Substitute into
	the template-head requirements of a template friend using only
	its outer arguments via outer_template_args.
	* cp-tree.h (outer_template_args): Declare.
	* pt.cc (outer_template_args): Define, factored out and
	generalized from ...
	(ctor_deduction_guides_for): ... here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-friend12.C: New test.
	* g++.dg/cpp2a/concepts-friend13.C: New test.

5 files changed, 70 insertions, 13 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 37eae03..247a827 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1350,11 +1350,12 @@ maybe_substitute_reqs_for (tree reqs, const_tree decl)
   if (DECL_UNIQUE_FRIEND_P (decl) && DECL_TEMPLATE_INFO (decl))
     {
       tree tmpl = DECL_TI_TEMPLATE (decl);
-      tree gargs = generic_targs_for (tmpl);
+      tree outer_args = outer_template_args (tmpl);
       processing_template_decl_sentinel s;
-      if (uses_template_parms (gargs))
+      if (PRIMARY_TEMPLATE_P (tmpl)
+	  || uses_template_parms (outer_args))
 	++processing_template_decl;
-      reqs = tsubst_constraint (reqs, gargs,
+      reqs = tsubst_constraint (reqs, outer_args,
 				tf_warning_or_error, NULL_TREE);
     }
   return reqs;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 541d760..1f4967c 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7056,6 +7056,7 @@ extern tree maybe_set_retval_sentinel		(void);
 extern tree template_parms_to_args		(tree);
 extern tree template_parms_level_to_args	(tree);
 extern tree generic_targs_for			(tree);
+extern tree outer_template_args			(tree);
 
 /* in expr.cc */
 extern tree cplus_expand_constant		(tree);
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e68c749..cbe5898 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -4958,6 +4958,28 @@ generic_targs_for (tree tmpl)
   return template_parms_to_args (DECL_TEMPLATE_PARMS (tmpl));
 }
 
+/* Return the template arguments corresponding to the template parameters of
+   TMPL's enclosing scope.  When TMPL is a member of a partial specialization,
+   this returns the arguments for the partial specialization as opposed to those
+   for the primary template, which is the main difference between this function
+   and simply using e.g. the TYPE_TI_ARGS of TMPL's DECL_CONTEXT.  */
+
+tree
+outer_template_args (tree tmpl)
+{
+  tree ti = get_template_info (DECL_TEMPLATE_RESULT (tmpl));
+  if (!ti)
+    return NULL_TREE;
+  tree args = TI_ARGS (ti);
+  if (!PRIMARY_TEMPLATE_P (tmpl))
+    return args;
+  if (TMPL_ARGS_DEPTH (args) == 1)
+    return NULL_TREE;
+  args = copy_node (args);
+  --TREE_VEC_LENGTH (args);
+  return args;
+}
+
 /* Update the declared TYPE by doing any lookups which were thought to be
    dependent, but are not now that we know the SCOPE of the declarator.  */
 
@@ -30081,16 +30103,8 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
 static tree
 ctor_deduction_guides_for (tree tmpl, tsubst_flags_t complain)
 {
-  tree type = TREE_TYPE (tmpl);
-  tree outer_args = NULL_TREE;
-  if (DECL_CLASS_SCOPE_P (tmpl)
-      && CLASSTYPE_TEMPLATE_INSTANTIATION (DECL_CONTEXT (tmpl)))
-    {
-      outer_args = copy_node (CLASSTYPE_TI_ARGS (type));
-      gcc_assert (TMPL_ARGS_DEPTH (outer_args) > 1);
-      --TREE_VEC_LENGTH (outer_args);
-      type = TREE_TYPE (most_general_template (tmpl));
-    }
+  tree outer_args = outer_template_args (tmpl);
+  tree type = TREE_TYPE (most_general_template (tmpl));
 
   tree cands = NULL_TREE;
 
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 0000000..9687be5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend12.C
@@ -0,0 +1,21 @@
+// PR c++/107853
+// { 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" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend13.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend13.C
new file mode 100644
index 0000000..3cc4505
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend13.C
@@ -0,0 +1,20 @@
+// Verify we substitute the correct outer template arguments
+// when instantiating a constrained template friend declared
+// inside a partial specialization.
+// { dg-do compile { target c++20 } }
+
+template<class U>
+  requires __is_same(int*, U)
+void f() { };
+
+template<class T>
+struct A;
+
+template<class T>
+struct A<T*> {
+  template<class U>
+    requires __is_same(T, U)
+  friend void f() { } // { dg-bogus "redefinition" }
+};
+
+template struct A<int*>;