When cross-compiling, CMake needs to find the build host's Qt6CoreTools. It
therefore prepends QT_HOST_PATH, which is /usr, to CMAKE_FIND_ROOT_PATH. The
problem is that CMAKE_FIND_ROOT_PATH is only a hint, not a definitive
location. Just below, CMake's find_package is usually told to look in
/usr/${CHOST}/usr/lib/cmake and /usr/lib/cmake when cross-compiling. Since
both of these are under /usr, it chooses the former instead of the latter.
It then ends up trying to execute non-native Qt binaries.

We can avoid this problem by setting CMAKE_FIND_ROOT_PATH to a more precise
location. All the Qt6 modules are installed under /usr/lib/cmake, represented
by the __qt_find_package_host_qt_path variable, so we can point it there.

find_package has two modes, module mode and config mode. No mode is
explicitly chosen in this case, so it tries both. In module mode, it would
use a module called FindQt6*.cmake, but no such module exists. It is
therefore safe to assume config mode, which involves the files under
/usr/lib/cmake.

See the isSameDirectoryOrSubDirectory() call in CMake's
cmFindCommon::RerootPaths() function for exactly where this goes wrong.

Chewi

https://bugs.gentoo.org/950314

--- a/cmake/QtConfig.cmake.in
+++ b/cmake/QtConfig.cmake.in
@@ -131,7 +131,7 @@
         set(__qt_backup_cmake_find_root_path "${CMAKE_FIND_ROOT_PATH}")
         list(PREPEND CMAKE_PREFIX_PATH "${__qt_find_package_host_qt_path}"
             ${_qt_additional_host_packages_prefix_paths})
-        list(PREPEND CMAKE_FIND_ROOT_PATH "${QT_HOST_PATH}"
+        list(PREPEND CMAKE_FIND_ROOT_PATH "${__qt_find_package_host_qt_path}"
             ${_qt_additional_host_packages_root_paths})
     endif()