fixes #1658

compiler/semtypes.nim

@@ -789,7 +789,7 @@ proc liftParamType(c: PContext, procKind: TSymKind, genericParams: PNode,
                                   @[newTypeS(paramType.kind, c)])
       result = addImplicitGeneric(typ)
     else:
-      for i in 0 .. <paramType.sons.len:
+      for i in 0 .. <paramType.len:
         if paramType.sons[i] == paramType:
           globalError(info, errIllegalRecursionInTypeX, typeToString(paramType))
         var lifted = liftingWalk(paramType.sons[i])

tests/objects/tobjloop.nim

@@ -0,0 +1,15 @@
+discard """
+  output: "is Nil false"
+"""
+# bug #1658
+
+type
+  Loop* = ref object
+    onBeforeSelect*: proc (L: Loop)
+
+var L: Loop
+new L
+L.onBeforeSelect = proc (bar: Loop) =
+  echo "is Nil ", bar.isNil
+
+L.onBeforeSelect(L)