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*>;