Can't use type aliases or conditional types resolving to Promise for async/await return types

TypeScript Version: 3.1

Search Terms:

• async promise alias
• async promise conditional

Code

type P<T> = Promise<T>

async function foo<T>(x: T): P<T> {
    return x;
}

async function bar<T>(x: T): T extends number ? Promise<number> : Promise<any> {
    return x;
}

Expected behavior:
No error.

Actual behavior:
Type 'P' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

Type 'T extends number ? Promise : Promise' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

Playground Link:
Link

Related Issues:

Start by compiling the two examples with TypeScript 3.1 and confirm the reported ES5/ES3 diagnostics. Trace the async-function return-type validation for type aliases and conditional types, then add regression coverage showing both examples compile without error.