# Compile an expression an return its result
	# `mtype` is the expected return type, pass null if no specific type is expected.
	fun expr(nexpr: AExpr, mtype: nullable MType): RuntimeVariable do
		var old = current_node
		current_node = nexpr

		var res = null
		if nexpr.mtype != null then
			res = nexpr.expr(self)
		end

		if res == null then
			# Untyped expression.
			# Might mean dead code or invalid code.
			# so aborts
			add_abort("FATAL: bad expression executed.")
			# and return a placebo result to please the C compiler
			if mtype == null then mtype = compiler.mainmodule.object_type
			res = null_instance

			self.current_node = old
			return res
		end

		if mtype != null then
			mtype = anchor(mtype)
			res = autobox(res, mtype)
		end

		current_node = old
		return res
	end