I try to compile Qt 6.11.x (0, 1, 2) for the iOS simulator. I managed it to run the configuration step, but compiling it failes. Here is the configuration I used:

/opt/Qt/6.11.2/Src/qtbase/configure -top-level LLVM_INSTALL_DIR=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain  \
-prefix /Users/user/SDKs/ios-sim-6.11.2 -extprefix /Users/user/SDKs/ios-sim-6.11.2 -release -platform macx-ios-clang \
-qt-host-path /Users/user/projects/Qt6.11.2_ios_sim/HostBuild -sdk iphonesimulator -DCMAKE_OSX_SYSROOT=iphonesimulator \
-skip-tests opengl -- -DQT_HOST_PATH_CMAKE_DIR=/Users/user/projects/Qt6.11.2_ios_sim/HostBuild/qtbase/lib/cmake \
-DCMAKE_OSX_DEPLOYMENT_TARGET=18 -DQT_OSX_ARCHITECTURES=arm64 -DCMAKE_OSX_ARCHITECTURES=arm64 \
-DOPENSSL_ROOT_DIR=/Users/user/SDKs/openssl/ssl_3/iossim -DCMAKE_THREAD_LIBS_INIT=-lpthread \
-DCMAKE_HAVE_THREADS_LIBRARY=1 -DCMAKE_USE_PTHREADS_INIT=1 -DTHREADS_PREFER_PTHREAD_FLAG=ON \
-DCMAKE_APPLE_ARCH_SYSROOTS=iphonesimulator

While the host build succeeded fine, I get the following error at the moment it starts to compile:

[...]
-- Configuration summary has been written to /Users/user/projects/Qt6.11.2_ios_sim/build/config.summary

Note: Using static linking will disable the use of dynamically loaded plugins. Make sure to import all needed static plugins, or compile needed modules into the library.
Note: Hunspell in Qt Virtual Keyboard is not enabled. Spelling correction will not be available.
Note: Hunspell in Qt Virtual Keyboard is not enabled. Spelling correction will not be available.

WARNING: QtPdf won't be built. The following configure errors were found:
 * Missing C++20 compiler support.
WARNING: QtWebEngine has warnings. The following configure warnings were found:
 * No openh264 library with version 2.4.1 or later. Using built-in one
 * No libavformat library with version 60.16.100 or later. Using built-in one
 * No libavcodec library with version 60.31.102 or later. Using built-in one
 * No libavutil library with version 58.29.100 or later. Using built-in one
 * No vpx library with version 1.10.0 or later. Using built-in one
 * No opus library with version 1.3.1 or later. Using built-in one
 * No icu library with version 70 or later. Using built-in one
 * No re2 library with version 11.0.0 or later. Using built-in one
 * No libtiff library with version 4.5.0 or later. Using built-in one
 * No libpng library with version 1.6.0 or later. Using built-in one
 * No libudev library with version  or later. Using built-in one
 * No harfbuzz library with version 4.3.0 or later. Using built-in one
 * No glib library with version 2.32.0 or later. Using built-in one
WARNING: QtWebEngine won't be built. The following configure errors were found:
 * AppleClang compiler is not supported.
 * Static build is not supported.
 * Build can be done only on Linux, Windows or macOS.
 * Missing C++20 compiler support.
WARNING: Not all build dependencies were found. Skipping building qtopenapi.
WARNING: The upstream OpenAPI generator CLI tool was not found. This doesn't affect the build of the generator plugin itself, but examples and tests will fail to build.

ERROR: detected a std::atomic implementation that fails for function pointers.

 Compile test outputs:

   TEST_atomicfptr output: 

Change Dir: '/Users/user/projects/Qt6.11.2_ios_sim/build/CMakeFiles/CMakeScratch/TryCompile-B1hynL'

Run Build Command(s): /opt/Qt/Tools/Ninja/ninja -v cmTC_9de0e
[1/2] /usr/bin/clang++ -DHAVE_atomicfptr  -std=c++20 -isysroot /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/Developer/Platforms/iPhoneSimulator.platform -Werror=unguarded-availability-new -arch arm64 -Xarch_arm64 -isysrootiphonesimulator -mios-simulator-version-min=18 -MD -MT CMakeFiles/cmTC_9de0e.dir/src.cxx.o -MF CMakeFiles/cmTC_9de0e.dir/src.cxx.o.d -o CMakeFiles/cmTC_9de0e.dir/src.cxx.o -c /Users/user/projects/Qt6.11.2_ios_sim/build/CMakeFiles/CMakeScratch/TryCompile-B1hynL/src.cxx
FAILED: CMakeFiles/cmTC_9de0e.dir/src.cxx.o 
/usr/bin/clang++ -DHAVE_atomicfptr  -std=c++20 -isysroot /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/Developer/Platforms/iPhoneSimulator.platform -Werror=unguarded-availability-new -arch arm64 -Xarch_arm64 -isysrootiphonesimulator -mios-simulator-version-min=18 -MD -MT CMakeFiles/cmTC_9de0e.dir/src.cxx.o -MF CMakeFiles/cmTC_9de0e.dir/src.cxx.o.d -o CMakeFiles/cmTC_9de0e.dir/src.cxx.o -c /Users/user/projects/Qt6.11.2_ios_sim/build/CMakeFiles/CMakeScratch/TryCompile-B1hynL/src.cxx
clang++: warning: no such sysroot directory: 'iphonesimulator' [-Wmissing-sysroot]
/Users/user/projects/Qt6.11.2_ios_sim/build/CMakeFiles/CMakeScratch/TryCompile-B1hynL/src.cxx:1:11: fatal error: 'atomic' file not found
    1 |  #include <atomic>
      |           ^~~~~~~~
1 error generated.
ninja: build stopped: subcommand failed.


ERROR: The OpenGL functionality tests failed! You might need to modify the OpenGL package search path by setting the OpenGL_DIR CMake variable to the OpenGL library's installation directory.

CMake Error at qtbase/cmake/QtBuildInformation.cmake:245 (message):
  Check the configuration messages for an error that has occurred.
Call Stack (most recent call first):
  qtbase/cmake/QtBuildInformation.cmake:64 (qt_configure_print_summary)
  qtbase/cmake/QtBaseTopLevelHelpers.cmake:114 (qt_print_feature_summary)
  qtbase/cmake/QtBaseTopLevelHelpers.cmake:93 (qt_internal_print_top_level_info)
  CMakeLists.txt:124 (qt_internal_top_level_end)


-- Configuring incomplete, errors occurred!
CMake Error at /opt/Qt/6.11.2/Src/qtbase/cmake/QtProcessConfigureArgs.cmake:1500 (message):
  CMake exited with code 1.


CMake Error at /opt/Qt/6.11.2/Src/cmake/QtIRProcessHelpers.cmake:173 (message):
  /opt/Qt/6.11.2/Src/qtbase/configure -top-level
  LLVM_INSTALL_DIR=/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain
  -prefix /Users/user/SDKs/ios-sim-6.11.2 -extprefix
  /Users/user/SDKs/ios-sim-6.11.2 -release -platform macx-ios-clang
  -qt-host-path /Users/user/projects/Qt6.11.2_ios_sim/HostBuild -sdk
  iphonesimulator -DCMAKE_OSX_SYSROOT=iphonesimulator -skip-tests opengl --
  -DQT_HOST_PATH_CMAKE_DIR=/Users/user/projects/Qt6.11.2_ios_sim/HostBuild/qtbase/lib/cmake
  -DCMAKE_OSX_DEPLOYMENT_TARGET=18 -DQT_OSX_ARCHITECTURES=arm64
  -DCMAKE_OSX_ARCHITECTURES=arm64
  -DOPENSSL_ROOT_DIR=/Users/user/SDKs/openssl/ssl_3/iossim
  -DCMAKE_THREAD_LIBS_INIT=-lpthread -DCMAKE_HAVE_THREADS_LIBRARY=1
  -DCMAKE_USE_PTHREADS_INIT=1 -DTHREADS_PREFER_PTHREAD_FLAG=ON
  -DCMAKE_APPLE_ARCH_SYSROOTS=iphonesimulator exited with status: 1

Call Stack (most recent call first):
  /opt/Qt/6.11.2/Src/cmake/QtTopLevelHelpers.cmake:35 (qt_ir_execute_process_and_log_and_handle_error)
  /opt/Qt/6.11.2/Src/cmake/QtTopLevelHelpers.cmake:57 (qt_tl_run_toplevel_configure)
  /opt/Qt/6.11.2/Src/cmake/QtTopLevelConfigureScript.cmake:16 (qt_tl_run_main_script)


Exiting because of previous errors!

It seems it doesn't like the atomic function pointers the way it's implemented.

Are my configuration settings wrong?
Can I work around this error?
Has somebody already compiled Qt 6.11 for iOS simulator? And if so, how were the settings?

I'm trying on a Mac Mini M1 with MacOS 27. The Mac OS is up to date.

A.T.