type ErrorBrand<Err extends string> = Readonly<{
  [key in Err]: void;
}>;
const booPonies = <T>(
  ponies: keyof T extends "pony" ? ErrorBrand<"No ponies allowed!"> : T
) => {};
booPonies("no ponies"); // Good
booPonies({ pony: "neigh" }); // ヽ(ಠ_ಠ)ノ