What is hydroxychloroquine, which Trump has asked India to release?

Hydroxychloroquine is a common anti-malaria drug now seen as a possible treatment for COVID-19. There have been no complete clinical trials to fully ascertain whether the drug, which can also treat rheumatoid arthritis, is an effective coronavirus treatment. US President Donald Trump called the unproven drug a "game-changer" against coronavirus after a small French study suggested it could help.