CanvasRenderingContext2D#drawImage overloading problem

[NekR]

This code:

var canvas = document.createElement('canvas');
var context = canvas.getContext('2d');
var drawThing:HTMLImageElement|HTMLCanvasElement;

context.drawImage(drawThing, 0, 0);

fails with error:

2345 Argument of type 'HTMLCanvasElement | HTMLImageElement' is not assignable to parameter of type 'HTMLVideoElement'.
  Type 'HTMLCanvasElement' is not assignable to type 'HTMLVideoElement'.

But I thought drawImage accepts one of HTMLCanvasElement | HTMLImageElement | HTMLVideoElement, isn't it?

[danquirk]

Unfortunately while drawImage does have 3 overloads which cover the three types you mention, that is not the same as actually accepting a union of those 3 types. See #1805 for some more discussion, these DOM APIs are certainly motivating examples for why we should consider the solution in that issue where we would special case single argument functions.