cleanup is not a function

[Rich-Harris]

If a deriver returns a non-falsy value, Svelte assumes it is a function, even if it isn't. This causes problems with e.g. async derivers:

const foo = writable(1);

// the deriver returns a promise
const bar = derived(foo, async ($foo, set) => {
  set($foo * 2);
});

Demo

[loilo]

This has created some headaches for me as well yesterday — not even because I used async functions but because I tried mixing up return value with set(value). It should probably be explicitely documented that accepting the set parameter is mutually exclusive with returning a derived value.