diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index bfd16e1ef62..169e6a62f5f 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -20041,7 +20041,16 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
   /* In a template parameter list, a type-parameter can be introduced
      by type-constraints alone.  */
   if (processing_template_parmlist && !placeholder)
-    return build_constrained_parameter (con, proto, args);
+    {
+      /* In a default argument we may not be creating new parameters.  */
+      if (parser->local_variables_forbidden_p & LOCAL_VARS_FORBIDDEN)
+	{
+	  /* If this assert turns out to be false, do error() instead.  */
+	  gcc_assert (tentative);
+	  return error_mark_node;
+	}
+      return build_constrained_parameter (con, proto, args);
+    }
 
   /* Diagnose issues placeholder issues.  */
   if (!flag_concepts_ts
diff --git a/gcc/testsuite/g++.dg/concepts/variadic6.C b/gcc/testsuite/g++.dg/concepts/variadic6.C
new file mode 100644
index 00000000000..0b386b0cd6d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/variadic6.C
@@ -0,0 +1,20 @@
+// PR c++/105268
+// { dg-do compile { target concepts } }
+
+template<typename> concept C_one = true;
+template<typename...> concept C_many = true;
+
+template<bool B> struct S { };
+
+template<typename T = S<C_one<int>>> void f();
+template<typename T = S<C_many<int>>> void g();
+
+void
+fn (auto a = S<C_one<int>>{})
+{
+}
+
+void
+fn2 (auto a = S<C_many<int>>{})
+{
+}