Return values of Promises are not checked in runtime

This is a:

• Bug Report

Which concerns (not really sure):

• flow-runtime
• babel-plugin-flow-runtime
• flow-runtime-validators

The following code does not raise any errors:

type Person = {
  name: string
}

function untrustedEndpoint(id: string): Promise<{ data: Person }> {
  return Promise.resolve(null) // <= wrong
}

untrustedEndpoint('id')

Currently we will only get the type assertion error once another function receives the return value as an argument, so a workaround is to:

type Person = {
  name: string
}

type Endpoint = { data: Person }

function untrustedEndpoint(id: string): Promise<Endpoint> {
  return Promise.resolve(null) // <= wrong
}

function processPersonNameFromEndpoint(response : Endpoint) {
  return response.data.name
}

untrustedEndpoint('id').then(processPersonNameFromEndpoint)

Raised by @gcanti at https://github.com/gcanti/io-ts/issues/4#issuecomment-285756198

[phpnode]

We can also skip this check in almost all cases in async functions - we’d only need it in the fairly rare case of returning a promise, ok, will add it.

[phpnode]

Not sure if returning a promise really is such a rare case (sometimes it’s superior than using async/await), but I understand where you’re coming from. Thanks!

I just mean in async function specifically:

async function foo (): Promise<string> {
  await bar();
  return Promise.resolve(null); // no inner type check here, but you'd usually await anyway
}

This does get a check:

function foo (): Promise<string> {
  return Promise.resolve(null); // throws
}