«فَلَوْ شَاءَ لَهَدَاكُمْ أَجْمَعِينَ»
Had God willed, He would have guided all of you.
The complement of «شاء» is unexpressed. The consequence «لهداكم» reveals that the assumed willing concerns their guidance. The intended relation is therefore familiar and recoverable rather than surprising.
The clause cannot be treated as an entirely undirected act of willing. The conditional consequence supplies the relevant content of what is supposed to have been willed.