[qtbase] Fix LNK2005 in consumers defining QT_DISABLE_DEPRECATED_BEFORE

[drdanz]

On Windows static builds, compat/removed_api.cpp emits strong symbol definitions for deprecated APIs. If consumers compile with a higher QT_DISABLE_DEPRECATED_BEFORE value, those same functions become inline definitions in consumer translation units, causing LNK2005 duplicate symbol errors.

Fix this in qtcoreexports.h (generated from cmake/modulecppexports.h.in) by adding a dedicated branch for static consumers that always makes QT_CORE_INLINE_SINCE, QT_CONSTEXPR_INLINE_SINCE and QT_CORE_INLINE_IMPL_SINCE produce non-inline.

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[Neumann-A]

Link to upstream ticket? Is this fixed in 6.11?

[drdanz]

Link to upstream ticket? Is this fixed in 6.11?

No upstream ticket yet, or at least I haven't found one, and I haven't opened one yet, because I'm still trying to figure out if this is the right fix. I don't think it is fixed in 6.11, but I haven't fully investigated.

[drdanz]

#50686 rebased over this PR passed all CI checks, therefore this seems to be working properly

[vicroms]

@drdanz please submit a ticket upstream with these fixes

[drdanz]

Done:

https://qt-project.atlassian.net/browse/QTBUG-145703
https://codereview.qt-project.org/c/qt/qtbase/+/727250

[vicroms]

We'll wait for the PR to be merged upstream.

[drdanz]

Apparently, setting a different QT_DISABLE_DEPRECATED_UP_TO for static builds is not supported in Qt, see the comments on the issue and on the patch, and also https://doc.qt.io/qt-6/qtdeprecationmarkers.html#QT_DISABLE_DEPRECATED_UP_TO

Therefore I'm afraid the patch won't get merged upstream.

This obviously does not work well with vcpkg sharing a single compiled library with all dependencies.

I still believe that the patch is correct, it simply disables building the "disabled" deprecated APIs in the consumer (in static builds only), that are currently compiled because they are inlined.

The function comments clearly say that "static builds treat everything as part of the library, so they never inline" therefore, for static consumers they should never be compiled, and should never be inlined.

The alternative to this patch is probably to patch ECM (and any other dependency that should have the same problem) to not set QT_DISABLE_DEPRECATED_UP_TO for static qt builds.