a.cpp

#include <stdio.h>

// libraries/AP_HAL_Linux
namespace HAL_Linux {
struct Mutex {
    Mutex() {
        printf("hello world Linux");
    }
};
}


// libraries/AP_HAL_PX4
namespace HAL_PX4 {
struct Mutex {
    Mutex() {
        printf("hello world PX4");
    }
};
}

namespace HAL {
    using Mutex = HAL_Linux::Mutex;
}

int main()
{
    HAL::Mutex mutex;
    return 0;
}