[Development] Proposing to officially allow C++20 types in Qt 6 ABIs

[Philippe]

foremost among them are coroutines

Note that on OSX / Clang, coroutines is still in <experimental/...

Philippe

On Thu, 27 Jan 2022 11:38:05 +0000
Marc Mutz <marc.mutz at qt.io> wrote:

Hi,
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> C++20 brings several new library features that would be great to use in the Qt API, foremost among them are coroutines and std::span. 
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> Yet, both of these are, in a sense, type-erasure techniques, and therefore most useful across ABI boundaries.
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> Traditionally, Qt has defined a set of minimum supported C++ compilers and whatever their lowest common denominator was, that's what  could be used in the Qt ABI, everything else, let's call it conditionally-supported, could only be used in inline API.
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> It is worth noting that on MSVC, inline API in exported classes already forms part of the ABI, and no-one seems to have cared.
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> I would therefore like to propose to soften our ABI guidelines such that we can officially use said conditionally-supported types also in the ABI. This would allow to create non-inline APIs that use std::span and coroutines without having to wait for all compilers to support them.
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> This is not such a big step: We already have that, de facto, in MSVC. The only difference would be that a C++20 project would require a C++20(or later)-build of Qt. The error upon failure to comply would be unspectacular: linker not finding a symbol in the Qt library.
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> Seeing as we have allowed C++latest types in our ABI de-facto in the past, I think it's time to allow it de-jure, too.
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> Discuss ??
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> Thanks,
> Marc
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> Marc Mutz
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