Type inference is overly strict when dealing w/ Union types and overloads

[nimish]

interface OverloadedMethod {
    (a:string): Object;
    (a:number): void;
}

const bar : string|number = 0;
const foo : OverloadedMethod = null;
foo(bar) // red squiggles here under bar: Argument of type 'string | number' is not assignable to 'string'. Type 'string | number' is not assignable to 'number'

This is a minimal example of where typescript complains that bar cannot unify with any of the individual methods on foo, even though it could reasonable deduce that since there's only one implementation of said method, that (a:string|number) : Object|void is a perfectly cromulent alternative.

Looks like this is a consequence of #5766 and it rears it head when dealing w/ Express routing's d.ts.

[RyanCavanaugh]

Please search for duplicates before logging new issues. #1805