Is Post Malone a rapper, rockstar or R&B singer? Trick question: the New York-born artist is whoever he wants to be, having proved himself remarkably versatile since the woozy melodies of his 2015 breakthrough hit “White Iverson”. In the 21 Savage collaboration “rockstar”, Post throws TVs out of hotel windows and shouts out Van Morrison, while on “Go Flex” he soundtracks a swaggy campfire sing-along, and he even sounds at home on “Fade”, a house-sampling Kanye West party-starter.