[bug] qt-6.10.1 build error on linux/clang with compiler.cppstd=20

[timblechmann]

Describe the bug

qt does not build with compiler.cppstd=20, but it needs gnu extensions. is there any way to force compilation with std=gnu++20 when compiler.cppstd=20 is in the profile? or can this configuration be rejected before having a compile error?

compare: https://qt-project.atlassian.net/browse/QTBUG-139280

How to reproduce it

No response

[memsharded]

Hi @timblechmann

Thanks for the feedback.
This wouldn't be a Conan bug, but maybe a request to improve the qt recipe in ConanCenter.

From the Conan point of view, you can always specify the cppstd you want for specific packages:

[settings]
....
compiler.cppstd=20
...
qt/*:compiler.cppstd=gnu20

You can do the same in command line too.

If qt/*:compiler.cppstd=gnu20 is what you were looking for, we can close the issue.
If not, I will transfer the ticket to conan-center-index repo. Thanks!

[timblechmann]

thanks for the quick reply. my bad for filing the ticket at the wrong location, the ticket should be for conan-center-index then. yes, i've worked around the compile error by using gnu20 instead of 20, but it would be great if the conan recipe could handle this gracefully (either by failing early or by mapping to gnu20 internally, if that's possible)

[memsharded]

No prob, transferred the ticket to conan-center-index

[jcar87]

We can fail early if that's helpful - this is something that has come up before. Conan by default assumes gnu extensions, as they are enabled by default by gcc itself, so we'd expect most users to continue with gnu17, gnu20 etc, unless they know for a fact they don't want them (but in that case, qt wont build)

[timblechmann]

Conan by default assumes gnu extensions

i've seen this when using conan_provider.cmake, which infers the cppstd from the running cmake process.
i.e.:

  set(CMAKE_CXX_STANDARD 20)
  set(CMAKE_CXX_EXTENSIONS ON) # if CMAKE_CXX_EXTENSIONS is not set explicitly, it won't enable the gnu20 extensions.

from the side of qt, i don't see how it affects the ABI: one can typically compile qt with one standard and application code with another (for shared libraries, this is guaranteed by qt, for static libraries there are only edge cases where it matters (iirc in the winrt runtime)).
so from the side of the conan recipe, forcing gnu extensions even (if they are not enabled) should be safe. otherwise a configure-time error would be great