variable name is_const -> ret_is_const
variable name is_const -> ret_is_const

`is_const` will get some strange messages in VS 2019
maybe it is conflicted with `std::is_const`
OrfeasZ/sol2,ThePhD/sol2,OrfeasZ/sol2,OrfeasZ/sol2,ThePhD/sol2,OrfeasZ/sol2,ThePhD/sol2,ThePhD/sol2