[Development] GCC 16 libstdc++ C++20 ABI break affects Qt C++17 builds, too

[Marc Mutz]

Hi,

GCC 16 changed the numerical value of std::partial_ordering::unordered. This is BiC, of course, but, as far as we know, libstd++ only breaks the C++20 ABI, not the C++17 one.

For better or worse, we have implemented Qt::partial_ordering as a binary-compatible C++17 replacement for C++20 std::partial_ordering (ditto the other ordering types, but, so far, no platform has broken ABI for those, yet). Binary-compatible means that we hard-code our numerical values to each stdlib's values. So there's no one Qt::partial_ordering::unordered value, it differs per stdlib, and we have tests that, in C++20 builds, make sure the two (Qt:: and std::) are of the same size and numerical value.

The idea was that the two are interchangeable both in BC and SC terms. Because our C++17 types are made BC with the C++20 std types, a C++17 build of Qt now inherits the issue for GCC 15 → 16.

We have two options, both of them are not optimal:

We can continue to maintain BC with the std types and break BC for Qt builds of GCC 15 and 16 or we maintain BC for Qt builds of GCC 15 and 16 and break BC with the std types.

We don't know which option is worse, so we'd like to get your feedback. OTOH, users may have come to depend on the BC between the Qt and std types and use them in their own ABIs. OTOH, users may expect that a  C++17 build of Qt is BC between GCC 15 and 16 because libstdc++ only broke BC for C++20 builds, not C++17 ones.

We're in particular interested in how Linux distributions intend to handle the issue. Are you shipping C++20 or C++17 builds of Qt? Or separate ones? How do you handle the C++20 BiC in libstdc++ for GCC 16?

Personally, I think fixing stdlib BiCs in Qt is a slippery slope. When we accepted std types in the Qt ABI, we accepted that a stdlib ABI break breaks Qt BC, too. The only reason we didn't see problems earlier was that we're coming out of a 15-year period of stability where no stdlib ABI was broken since C++11's container changes. So it comes as a surprise now, even though we should have anticipated it all the time.

Whatddayathink?

Thanks,
Marc


--
Marc Mutz <marc.mutz at qt.io><mailto:marc.mutz at qt.io> (he/his)
Principal Software Engineer

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