Qt6: change variable setting from FEATURE_cxx to QT_FEATURE_cxx

[weynaa]

Summary

Changes to recipe: qt/6.x.x

Motivation

See #25651
With this change the CMAKE_CXX_STANDARD property is propagated correctly to the Qtbase submodule and most likely others aswell.

D:\dev\conan-center-index\recipes\qt\6.x.x (fix-qt-cxx-flag -> fork)
λ conan create . --version=6.7.3 -s compiler.cppstd=20
....
qt/6.7.3: RUN: cmake -G "Ninja" -DCMAKE_TOOLCHAIN_FILE="D:/conan-cache/b/qtd417356601dd9/b/build/generators/conan_toolchain.cmake" -DCMAKE_INSTALL_PREFIX="D:/conan-cache/b/qtd417356601dd9/p" -DCMAKE_TRY_COMPILE_CONFIGURATION="Release" -DQT_USE_VCPKG="OFF" -DCMAKE_POLICY_DEFAULT_CMP0091="NEW" -DCMAKE_BUILD_TYPE="Release" "D:/conan-cache/qt9bbf649b16297/s/src"
conanvcvars.bat: Activating environment Visual Studio 17 - amd64 - winsdk_version=None - vcvars_ver=14.4
[vcvarsall.bat] Environment initialized for: 'x64'
-- Using Conan toolchain: D:/conan-cache/b/qtd417356601dd9/b/build/generators/conan_toolchain.cmake
-- Conan toolchain: Setting CMAKE_MSVC_RUNTIME_LIBRARY=$<$<CONFIG:Release>:MultiThreadedDLL>
-- Conan toolchain: C++ Standard 20 with extensions OFF
-- Conan toolchain: Setting BUILD_SHARED_LIBS = OFF
-- The CXX compiler identification is MSVC 19.41.34123.0
-- The C compiler identification is MSVC 19.41.34123.0
-- The ASM compiler identification is MSVC
-- Found assembler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.41.34120/bin/Hostx64/x64/cl.exe
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.41.34120/bin/Hostx64/x64/cl.exe - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: C:/Program Files/Microsoft Visual Studio/2022/Community/VC/Tools/MSVC/14.41.34120/bin/Hostx64/x64/cl.exe - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Checking dependencies of submodule 'qtbase'
-- Configuring submodule 'qtbase'
-- [QtBase] CMAKE_BUILD_TYPE was already explicitly set to: 'Release'
            -DCMAKE_C_FLAGS=/DWIN32 /D_WINDOWS
            -DCMAKE_C_FLAGS_DEBUG=/Zi /Ob0 /Od /RTC1
            -DCMAKE_C_FLAGS_RELEASE=/O2 /Ob2 /DNDEBUG
            -DCMAKE_C_FLAGS_RELWITHDEBINFO=/Zi /O2 /Ob1 /DNDEBUG
            -DCMAKE_CXX_FLAGS=/DWIN32 /D_WINDOWS /EHsc
            -DCMAKE_CXX_FLAGS_DEBUG=/Zi /Ob0 /Od /RTC1
            -DCMAKE_CXX_FLAGS_RELEASE=/O2 /Ob2 /DNDEBUG
            -DCMAKE_CXX_FLAGS_RELWITHDEBINFO=/Zi /O2 /Ob1 /DNDEBUG
            -DCMAKE_EXE_LINKER_FLAGS=/machine:x64
            -DCMAKE_TOOLCHAIN_FILE=D:/conan-cache/b/qtd417356601dd9/b/build/generators/conan_toolchain.cmake
            -DCMAKE_C_STANDARD=11
            -DCMAKE_C_STANDARD_REQUIRED=ON
            -DCMAKE_CXX_STANDARD=20
            -DCMAKE_CXX_STANDARD_REQUIRED=ON
            -DCMAKE_MODULE_PATH:STRING=D:/conan-cache/qt9bbf649b16297/s/src/qtbase/cmake/platforms

-- Configuration summary shown below. It has also been written to D:/conan-cache/b/qtd417356601dd9/b/build/config.summary
-- Configure with --log-level=STATUS or higher to increase CMake's message verbosity. The log level does not persist across reconfigurations.

-- Configure summary:

Building for: win32-msvc (x86_64, CPU features: )
Compiler: msvc 19.41.34123.0
Build options:

Details

Rename the defined cmake configuration variables for the c++ standard from FEATURE_cxx to QT_FEATURE_cxx. As far as I see it cannot propagate the CXX_STANDARD correctly with the previous name: https://github.com/qt/qtbase/blob/bab1fd8fc3f8d6af5ad4b603c0dabdca434759a4/cmake/QtFlagHandlingHelpers.cmake#L355-L368.

This flag for cppstd was originally added in #15098, @ericLemanissier I wonder if I missed something in my testing or if there was an upstream change in Qt that introduced or changed this. As far as i see, in version 6.4.0 it was also already named QT_FEATURE_cxx20 and a while before that.

---

• Read the contributing guidelines
• Checked that this PR is not a duplicate: list of PRs by recipe
• Tested locally with at least one configuration using a recent version of Conan

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[ericLemanissier]

Hi, I just followed documentation https://github.com/qt/qtbase/blob/bab1fd8fc3f8d6af5ad4b603c0dabdca434759a4/cmake/configure-cmake-mapping.md?plain=1#L58
all features are documented to be passed as -DFEATURE_foo=BAR. You should report the problem on https://bugreports.qt.io/

[weynaa]

Hi, Thanks for the link to the documentation. I agree i guess the user should use the FEAUTURE_xxx values without the QT_ prefix as that is supposed to be internal - still seems to be a bug from Qt though because the FEATURE_cxx20 variable does not propagate into the QT_FEATURE_cxx20 variable for some reason.

[valgur]

I think this PR still has merit until the issue has been fixed upstream, so please don't close. I would set both the QT_FEATURE_... and FEATURE_ variables in the CMakeToolchain as a workaround.

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[weynaa]

I think this PR still has merit until the issue has been fixed upstream, so please don't close. I would set both the QT_FEATURE_... and FEATURE_ variables in the CMakeToolchain as a workaround.

I opened a bug report with qt https://bugreports.qt.io/browse/QTBUG-126161. I got confused with the configure log because it wrongfully shows CXX_STANDARD=17 when you only set the FEATURE_cxx20 flag, but with the QT_FEATURE_cxx20 it shows the correct entry in the build log.

However, the actual -std:c++20 flag sent to the compiler looks to be unaffected by it after checking by just compiling qtbase verbose. Initially, i still got linkage errors as described in #15001 even with the compiler.cppstd=20 setting. I think that was caused by me being not careful and accidentally downloading an official package from conan-center and probabaly some other cached package/buildtree was used for my project:

> conan install --requires=qt/6.7.3 -s compiler.cppstd=20 -u
Profile host:
[settings]
arch=x86_64
build_type=Release
compiler=msvc
compiler.cppstd=20
compiler.runtime=dynamic
compiler.runtime_type=Release
compiler.version=194
os=Windows
[options]
with_qt=True
[conf]


Profile build:
[settings]
arch=x86_64
build_type=Release
compiler=msvc
compiler.cppstd=20
compiler.runtime=dynamic
compiler.runtime_type=Release
compiler.version=194
os=Windows
[conf]

....

qt/6.7.3: Found compatible package 'd096538b2a88cc969d69186b84eb615b739c2ed3': compiler.cppstd=17, compiler.version=193

Maybe there is a different problem here though: cppstd=17 and cppstd=20 are not ABI compatible in MSVC if you use coroutines (https://devblogs.microsoft.com/oldnewthing/20230111-00/?p=107694) but the recipie still thinks its compatible here and downloads a binary package online. If desired i can open a separate issue for that.

I was able to solve my issue by doing a clean build:

> conan remove qt*
> conan install --requires=qt/6.7.3 --build missing --build qt* -s compiler.cppstd=20

Feel free to close if no longer relevant.