async function return type checking is too strict #6027

// Three functions and their return types
function ordinaryFunction() { return 42; }      // returns number
function* generatorFunction() { yield 42; }     // returns IterableIterator<number>
async function asyncFunction() { return 42; }   // returns Promise<number>

// Assign result to compatible interface type - behaviour is consistent
var o1: number = ordinaryFunction();                     // OK
var o2: any = ordinaryFunction();                        // OK
var g1: Iterable<number> = generatorFunction();          // OK
var g2: any = generatorFunction();                       // OK
var a1: PromiseLike<number> = asyncFunction();           // OK
var a2: any = asyncFunction();                           // OK

// Annotate result with compatible interface type - behaviour is inconsistent
function of1(): number { return 42; }                    // OK
function of2(): any { return 42; }                       // OK
function* gf1(): Iterable<number> { yield 42; }          // OK
function* gf2(): any { yield 42; }                       // OK
async function af1(): PromiseLike<number> { return 42; } // ERROR
async function af2(): any { return 42; }                 // ERROR