redef fun lookup_fixed(mmodule, resolved_receiver)
	do
		var t = mtype.lookup_fixed(mmodule, resolved_receiver)
		return t
	end

	# A VT is fixed when:
	# * the VT is (re-)defined with the annotation `is fixed`
	# * the receiver is an enum class since there is no subtype that can
	#   redefine this virtual type
	redef fun lookup_fixed(mmodule: MModule, resolved_receiver: MType): MType
	do
		assert not resolved_receiver.need_anchor
		resolved_receiver = resolved_receiver.undecorate
		assert resolved_receiver isa MClassType # It is the only remaining type

		var prop = lookup_single_definition(mmodule, resolved_receiver)
		var res = prop.bound
		if res == null then return new MErrorType(model)

		# Recursively lookup the fixed result
		res = res.lookup_fixed(mmodule, resolved_receiver)

		# For a fixed VT, return the resolved bound
		if prop.is_fixed then return res

		# For a enum receiver return the bound
		if resolved_receiver.mclass.kind == enum_kind then return res

		return self
	end

	# A PT is fixed when:
	# * The `resolved_receiver` is a subclass of `self.mclass`,
	#   so it is necessarily fixed in a `super` clause, either with a normal type
	#   or with another PT.
	#   See `resolve_for` for examples about related issues.
	redef fun lookup_fixed(mmodule: MModule, resolved_receiver: MType): MType
	do
		assert not resolved_receiver.need_anchor
		resolved_receiver = resolved_receiver.undecorate
		assert resolved_receiver isa MClassType # It is the only remaining type
		var res = self.resolve_for(resolved_receiver.mclass.mclass_type, resolved_receiver, mmodule, false)
		return res
	end

	# Efficiently returns `mtype.lookup_fixed(mmodule, resolved_receiver).as_nullable`
	redef fun lookup_fixed(mmodule, resolved_receiver)
	do
		var t = super
		if t == mtype then return self
		return t.as_nullable
	end

	# Efficiently returns `mtype.lookup_fixed(mmodule, resolved_receiver).as_notnull`
	redef fun lookup_fixed(mmodule, resolved_receiver)
	do
		var t = super
		if t == mtype then return self
		return t.as_notnull
	end