Error: mutable method isolated.graphics.g3d.model.Model.begin is not callable using a const object

[Begah via Digitalmars-d-learn]

On Monday, 6 June 2016 at 21:09:41 UTC, Alex Parrill wrote:
> On Monday, 6 June 2016 at 20:40:12 UTC, Begah wrote:
>> I have a pretty weird error : Error: mutable method 
>> isolated.graphics.g3d.model.Model.begin is not callable using 
>> a const object
>>
>> [...]
>
> It may infer const from the type of `this.instance`, which may 
> be further modified if the method you are running this in is 
> `const`. If you explicitly specify the type of the foreach loop 
> variables, does it work?

Seems that it does :

ModelInstance[][ModelType] instances;

void render() {
	checkError();
	shader.bind();

	camera.update();
	shader.uniform("uView", camera.viewMatrix);
	shader.uniform("uProjection", camera.projectionMatrix);

	foreach(ref ModelType model, ref instances; this.instances) {
		model.begin();
		foreach(instance; instances) {
			instance.render();
		}
		model.end();
	}

	shader.unbind();
	checkError();
}

Adding the type to the loop gives me the error : index must be 
type const(Handle), not Handle.
So why does it think that from the associative array that 
ModelType should be const?
Does the key of a associative array will be const if the 
reference is requested?