#if defined(__has_include)
    /*
        Notice that we trust our configure tests more than __has_include(),
        notably the latter can return true even if the header exists but isn't
        actually usable, as it happens with <type_traits> in non C++11 mode.
        So if configure already detected at least one working alternative,
        just use it.
     */
    #if !defined(HAVE_TYPE_TRAITS) && !defined(HAVE_TR1_TYPE_TRAITS)
        #if __has_include(<type_traits>)
            #define HAVE_TYPE_TRAITS
        #elif __has_include(<tr1/type_traits>)
            #define HAVE_TR1_TYPE_TRAITS
        #endif
    #endif