# Check that `sub` is a subtype of `sup`.
	# If `sub` is not a valid suptype, then display an error on `node` and return `null`.
	# If `sub` is a safe subtype of `sup`, then return `sub`.
	# If `sub` is an unsafe subtype (i.e., an implicit cast is required), then return `sup`.
	#
	# The point of the return type is to determinate the usable type on an expression when `autocast` is true:
	# If the suptype is safe, then the return type is the one on the expression typed by `sub`.
	# Is the subtype is unsafe, then the return type is the one of an implicit cast on `sup`.
	fun check_subtype(node: ANode, sub, sup: MType, autocast: Bool): nullable MType
	do
		if self.is_subtype(sub, sup) then return sub
		if autocast and self.is_subtype(sub, self.anchor_to(sup)) then
			# FIXME workaround to the current unsafe typing policy. To remove once fixed virtual types exists.
			#node.debug("Unsafe typing: expected {sup}, got {sub}")
			return sup
		end
		if sup isa MErrorType then return null # Skip error
		if sub.need_anchor then
			var u = anchor_to(sub)
			self.modelbuilder.error(node, "Type Error: expected `{sup}`, got `{sub}: {u}`.")
		else
			self.modelbuilder.error(node, "Type Error: expected `{sup}`, got `{sub}`.")
		end
		return null
	end