Confusing error for custom Promise type

[ghost]

TypeScript Version: nightly (2.5.0-dev.20170629)

Code

export declare class TPromise<T> {
	constructor(executor: (resolve: (value?: T /*| PromiseLike<T>*/) => void, reject: (reason?: any) => void) => void);

	// Copy-pasted from PromiseLike:
	then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>;
}

async function f(resource: any): TPromise<number> { return 0; }

Expected behavior:

Complains that constructor is not compatible with Promise's constructor. (Uncomment | PromiseLike<T> and there is no compile error.)

Actual behavior:

Complains that then is incompatible. But it's identical to the one in PromiseLike.

src/a.ts(8,34): error TS1055: Type 'typeof TPromise' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
src/a.ts(8,34): error TS1055: Type 'typeof TPromise' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.
  Type 'TPromise<T | PromiseLike<T>>' is not assignable to type 'PromiseLike<T>'.
    Types of property 'then' are incompatible.
      Type '<TResult1 = T | PromiseLike<T>, TResult2 = never>(onfulfilled?: (value: T | PromiseLike<T>) => TR...' is not assignable to type '<TResult1 = T, TResult2 = never>(onfulfilled?: (value: T) => TResult1 | PromiseLike<TResult1>, on...'.
        Types of parameters 'onfulfilled' and 'onfulfilled' are incompatible.
          Types of parameters 'value' and 'value' are incompatible.
            Type 'T | PromiseLike<T>' is not assignable to type 'T'.
              Type 'PromiseLike<T>' is not assignable to type 'T'.

Discovered in microsoft/vscode#30216

[HunderlineK]

Possibly related:

type FindByToken = Promise<IUserDocument | null>;

UserSchema.statics.findByToken = async function(token: string): FindByToken {

Will complain,

but

UserSchema.statics.findByToken = async function(token: string): Promise<IUserDocument | null> {

will be fine

[rbuckton]

@HunderlineK If you are targeting ES5 then you cannot use a type alias in the return type of an async function as the return type must resolve to a constructor in the value space, not just the type space.

[RyanCavanaugh]

Doesn't seem like anyone else has encountered this