Not assignable to Promise

[felixfbecker]

[ts] Type 'Bluebird<Model[]>' is not assignable to type 'Promise<User[]>'.
       Types of property 'then' are incompatible.
         Type '{ <U1, U2>(onFulfill: (value: Model[]) => U1 | Thenable<U1>, onReject: (error: any) => U2 | Thena...' is not assignable to type '{ <TResult1, TResult2>(onfulfilled: (value: User[]) => TResult1 | PromiseLike<TResult1>, onreject...'.
           Type 'Bluebird<any>' is not assignable to type 'Promise<any>'.
             Property '[Symbol.toStringTag]' is missing in type 'Bluebird<any>'.

This prevents Sequelize users from writing methods with a Promise return type and returning a bluebird promise.

types/sequelize#78

[blakeembrey]

I think this is reasonable, to be honest, especially if it matches the actual code (E.g. no symbols in Bluebird). What people should be using is PromiseLike for input arguments so they can accept anything that looks like a promise and the return types remain correct.

[felixfbecker]

I agree. A user can type as PromiseLike or the actual Bluebird promise.

[rokoroku]

I forgot to mention it:
The remain problem is Typescript doesn't allow PromiseLike return type for async functions.

[ts] The return type of an async function or method must be the global Promise<T> type.

[unional]

Relevant topic: microsoft/TypeScript#5911

[unional]

The most relevant comment: microsoft/TypeScript#5911 (comment)
microsoft/TypeScript#5911 (comment)
microsoft/TypeScript#5911 (comment)
microsoft/TypeScript#6631

[blakeembrey]

@rokoroku That's correct and should be unrelated. Use the global Promise and not bluebird for the return type of an async function (it'll always be converted at runtime to the global instance).