somePromise.catch or .then(null, rejectFn) should preserve type of original promise but doesn't #4903
Description
The fundamental point is that the catch of a promise should NOT change the type of that promise unless it returns something different. Returning something different is relatively rare because the activation of the catch should be rare.
Because catch is sugar over then(null, rejectFn), that form has the same issue.
Example 1: foo returns Promise<Hero>
foo(){
return Promise.resolve(new Hero('foo'));
}
Example 2: foo returns a Promise<{}> ! This is AWFUL
foo(){
return Promise.resolve(new Hero('foo'))
// Pick any of these to see the problem
.catch()
.catch(e => {})
.then(null, e => {})
}
Example 3: foo returns a Promise<{}> ! This is AWFUL TOO
foo(){
return Promise.resolve(new Hero('foo'))
// Pick either of these to see the problem
.catch(e => Promise.reject(e))
.then(null, e => Promise.reject(e))
}
Example 4: foo returns a Promise<Hero> ... but this is baroque
foo(){
return Promise.resolve(new Hero('foo'))
.then(_ => _, e => Promise.reject(e));
}
The problem is in the typings file I think. I contend that that catch or then(null, rejectFn) should be typed to return a Promise<Hero>.