Function with empty object type parameter assignable to functions accepting more specific type

[SimonCJacobs]

TypeScript Version: 2.4.1

This code shows the error that isn't picked up:

var numToString: ( x: number ) => string
var objToString = ( x: {} ) => x.toString();
numToString = objToString; // No compiler error, but should be: Type '{}' is not assignable to type 'number'

[kitsonk]

Event with weak type detection in 2.4.1 an empty object {} in a argument position essentially equates to any. You would need something else, like object or { [key: string]: value; } to make it not assignable. I am not 100% sure of the inner workings of why this is with the type system, but it has to do with arguments being checked in a co-variant manner, I believe.

[ghost]

{} is the type of all values except null and undefined. So, const x: {} = 0; is valid.
Since objToString will accept a number, it's valid to use objToString where a numToString is expected.
Unfortunately, it's not valid to assign objToString = numToString, since numToString can't accept non-numbers but objToString is supposed to; but TypeScript will allow it because functions are checked bivariantly.
See also: #10717, #12498, #9825, #16735, #14973