Bug#1087385: [Pkg-cmake-team] Bug#1087385: cmake 3.31.0-1 breaks Qt6Qml builds

[Patrick Franz <deltaone@debian.org>]

Hi Brad,

On Fri, 15 Nov 2024 08:42:16 -0500 Brad King <brad.king@kitware.com> 
wrote:
[...]
> Perhaps `Qt6QmlTargets` should be split by upstream to move the 
> unlikely target into a dedicated `Qt6QmlPrivateTargets` file.  
> Meanwhile, here is a workaround one could apply in Debian packaging:
> 
> One could patch `Qt6QmlConfig.cmake` to, after loading 
> `Qt6QmlTargets.cmake`, modify the `Qt6::QmlPrivate` target's 
> `INTERFACE_INCLUDE_DIRECTORIES` property to remove the entries 
> corresponding to `qt6-declarative-private-dev` if those directories do 
> not exist.  That will hide the dependency from the CMake diagnostic.  
> If an application really needs the headers, the error will occur 
> during its compilation instead.

Thank you for that suggestion. I tried it, but unfortunately it only 
solves half the problem. While we're not looking for 
"/usr/include/x86_64-linux-gnu/qt6/QtQml/6.7.2" any more, Qt::QmlPrivate 
is linked against Qt::CorePrivate which means we need the private 
headers for qt6-base found in qt6-base-private-dev. That package would 
then need to be added manually to the build dependencies.
We could of course do a similar thing and check for 
"/usr/include/x86_64-linux-gnu/qt6/QtCore/6.7.2" and not link against 
Qt::CorePrivate if those directories do not exist. But it feels like a 
super-ugly hack and I cannot shake the feeling that I'll be going down 
the rabbit hole here (not your fault of course).
What is complicating matters is that both Qt6QmlConfig.cmake and 
Qt6QmlTargets.cmake do not exist explicitly in the qtdeclarative source, 
but are instead generated files from a template in qtbase. So any change 
there would affect every single Qt package and I honestly cannot judge 
the effect of such a change.

Sadly, it seems that we can only choose the lesser evil here and have to 
depend on the private-dev package(s) for the time being until Qt 
upstream comes up with a better solution unless anyone has a better 
idea.


-- 
Med vänliga hälsningar

Patrick Franz