# Check the visibility of `mtype` as an element of the signature of `mpropdef`.
	fun check_visibility(node: ANode, mtype: MType, mpropdef: MPropDef)
	do
		var mmodule = mpropdef.mclassdef.mmodule
		var mproperty = mpropdef.mproperty

		# Extract visibility information of the main part of `mtype`
		# It is a case-by case
		var vis_type: nullable MVisibility = null # The own visibility of the type
		var mmodule_type: nullable MModule = null # The original module of the type
		mtype = mtype.undecorate
		if mtype isa MClassType then
			vis_type = mtype.mclass.visibility
			mmodule_type = mtype.mclass.intro_mmodule
		else if mtype isa MVirtualType then
			vis_type = mtype.mproperty.visibility
			mmodule_type = mtype.mproperty.intro_mclassdef.mmodule
		else if mtype isa MParameterType then
			# nothing, always visible
		else if mtype isa MNullType then
			# nothing to do.
		else if mtype isa MBottomType then
			# nothing to do.
		else if mtype isa MErrorType then
			# nothing to do.
		else
			node.debug "Unexpected type {mtype}"
			abort
		end

		if vis_type != null then
			assert mmodule_type != null
			var vis_module_type = mmodule.visibility_for(mmodule_type) # the visibility of the original module
			if mproperty.visibility > vis_type then
				error(node, "Error: the {mproperty.visibility} property `{mproperty}` cannot contain the {vis_type} type `{mtype}`.")
				return
			else if mproperty.visibility > vis_module_type then
				error(node, "Error: the {mproperty.visibility} property `{mproperty}` cannot contain the type `{mtype}` from the {vis_module_type} module `{mmodule_type}`.")
				return
			end
		end

		# No error, try to go deeper in generic types
		if node isa AType then
			for a in node.n_types do
				var t = a.mtype
				if t == null then continue # Error, thus skipped
				check_visibility(a, t, mpropdef)
			end
		else if mtype isa MGenericType then
			for t in mtype.arguments do check_visibility(node, t, mpropdef)
		end
	end