error TS1057, when resolving an `async` function with a

[eschwartz]

TypeScript Version: 2.2.0-dev.20161221

Code

Use Case A: Fails with async keyword, and any lookup type

interface SomeObj {
  stringProp: string;
  anyProp: any;
}

async function getAnyProp<TObj extends SomeObj>(obj:TObj):Promise<TObj['anyProp']> {
  return Promise.resolve(obj.anyProp);
}

Expected behavior:

Code should compile successfully

Actual behavior:

Code does not compile. Emits an error:

(6,16): error TS1057: An async function or method must have a valid awaitable return type.

Where (6,16) is the location of getAnyProp

Use case B: Succeeds without async keyword

Note that the code will compile if you remove the async keyword:

function getAnyProp<TObj extends SomeObj>(obj:TObj):Promise<TObj['anyProp']> {
  return Promise.resolve(obj.anyProp);
}
// compiles successfully

Use case C: Fails with async keyword, and generic passed to Promise.resolve

I also get a strange error if I pass a generic to Promise.resolve:

async function getAnyProp<TObj extends SomeObj>(obj:TObj):Promise<TObj['anyProp']> {
  return Promise.resolve<TObj['anyProp']>(obj.anyProp);
}

Results in:

(6,16): error TS1057: An async function or method must have a valid awaitable return type.
(7,10): error TS1059: Return expression in async function does not have a valid callable 'then' member.

Again, if I remove the async keyword, this example compiles successfully.

Use Case D: Succeeds with string lookup type

The issue seems only to occur if the resolved lookup type is of type any.

This code, for example, compile successfully.

interface SomeObj {
  stringProp: string;
  anyProp: any;
}

async function getStringProp<TObj extends SomeObj>(obj:TObj):Promise<TObj['stringProp']> {
  return Promise.resolve<TObj['stringProp']>(obj.anyProp);
}

It will also compile successfully with number, boolean, void, or {}.

Use Case E: Succeeds without generic

If I do not use generics, the code compiles successfully.

For example:

interface SomeObj {
  anyProp: any;
}

async function getAnyProp():Promise<SomeObj['anyProp']> {
  return Promise.resolve<SomeObj['anyProp']>('foo');
}

---

Thank you for your help! I'm really loving the new lookup types.

[rbuckton]

This is due to the fact that an IndexedType in our type system that points to any does not itself have the TypeFlags.Any flag, so it fails one of the conditions for checking for a "thenable" type. However, when we compare the type to Thenable, our type checking logic treats TObj['anyProp'] as any, which is why it is failing.

I'm testing a fix for this now.

[eschwartz]

Wow, you guys rock.