# Collect all types used in covariant and contravariant positions.
	#
	# The collect visits all classes and properties of `mainmodule` and its imported modules.
	#
	# After the visit, the attributes of `self` are filled.
	fun collect(mainmodule: MModule)
	do
		for m in mainmodule.in_importation.greaters do
			for cd in m.mclassdefs do
				if cd.is_intro then
					pts.add_all(cd.mclass.mparameters)
					var a = cd.mclass.arity
					if a == 0 then
						cpt_class.inc("non generic")
					else if a == 1 then
						cpt_class.inc("with 1 formal type parameter")
					else
						cpt_class.inc("with {a} formal type parameters")
					end
				end
				for t in cd.supertypes do
					# Supertype (covariant)
					if t.need_anchor then covar_classes.add(t)
				end
				for pd in cd.mpropdefs do
					if exclude_private and pd.mproperty.visibility <= private_visibility then continue
					if pd isa MMethodDef then
						# Parameters (contravariant)
						for p in pd.msignature.mparameters do
							var t = p.mtype.undecorate
							if not t.need_anchor then
								# OK
							else if t isa MParameterType then
								contravar_pt.add(t)
							else if t isa MVirtualType then
								# TODO?
							else if t isa MClassType then
								contravar_classes.add(t)
							else
								abort
							end
						end
						# Return (covariant)
						var t = pd.msignature.return_mtype
						if t != null and t.need_anchor then
							t = t.undecorate
							if t isa MParameterType then
								covar_pt.add(t)
							else if t isa MVirtualType then
								# TODO?
							else if t isa MClassType then
								covar_classes.add(t)
							else
								abort
							end
						end
					else if pd isa MAttributeDef then
						# Attribute (invariant)
						var t = pd.static_mtype
						if t != null and t.need_anchor then
							t = t.undecorate
							if t isa MParameterType then
								covar_pt.add t
								contravar_pt.add t
							else if t isa MVirtualType then
								# TODO?
							else if t isa MClassType then
								covar_classes.add(t)
								contravar_classes.add(t)
							else
								abort
							end
						end
					else if pd isa MVirtualTypeDef then
						# Virtual type bound (covariant)
						var t = pd.bound
						if t != null and t.need_anchor then
							t = t.undecorate
							if t isa MParameterType then
								covar_pt.add t
							else if t isa MVirtualType then
								# TODO?
							else if t isa MClassType then
								covar_classes.add(t)
							else
								abort
							end
						end
					end
				end
			end
		end
	end