When debugging a .qmlproject it is possible to enable the "C++" debugger checkbox in the project run settings. The following warning will be shown:

The preferred debugger engine for debugging binaries of type 'unknown-unknown-unknown-unknown-unknown' is not available.
The debugger engine 'Gdb engine' will be used as a fallback.
Details: The CDB debug engine does not support the unknown-unknown-unknown-unknown-unknown ABI.

Since this is not supported, the checkbox should be disabled.

To debug mixed applications, use the Debug/External Executable or create a 2nd run config for the project.