fix #6462

tests/concepts/t6462.nim

@@ -0,0 +1,23 @@
+discard """
+  output: "true"
+"""
+
+import future
+
+type
+  FilterMixin*[T] = ref object
+    test*:      (T) -> bool
+    trans*:     (T) -> T
+
+  SeqGen*[T] = ref object
+    fil*:     FilterMixin[T]
+  
+  WithFilter[T] = concept a
+    a.fil is FilterMixin[T]
+
+proc test*[T](a: WithFilter[T]): (T) -> bool =
+  a.fil.test
+
+var s = SeqGen[int](fil: FilterMixin[int](test: nil, trans: nil))
+echo s.test() == nil
+