On Unix 64-bit platforms (LP64), compilers will use long and unsigned long as the underlying type of 64-bit enumerations without explicit type, such as: enum E { V = 0x1'0000'0000 }; User code may also explicitly choose std::int64_t or std::uint64_t, which on those platforms map to long and unsigned long, respectively. Likewise std::ptrdiff_t and std::size_t, which are the same. This means we do allow streaming of enums with explicit long and unsigned long types, such as: enum ELong : long { A, B, C }; which change sizes between platforms. [ChangeLog][QtCore][QDataStream] QDataStream now supports streaming enumerations whose underlying type is long or unsigned long (like std::int64_t, std::uint64_t, ptrdiff_t, size_t on 64-bit Unix platforms). Fixes: QTBUG-86424 Change-Id: I34ef9422a252ce3df468fffd4ba074cddea20005 Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>