The spell Expeditious Retreat requires concentration, and reads:
When you cast this spell, and then as a bonus action on each of your turns until the spell ends, you can take the Dash action.
Assume that a character has cast Expeditious Retreat on themselves in a previous turn. This turn, they have used all of their normal movement, and that character starts the Bonus Action Dash and uses some but not all of the movement granted by this Dash. If concentration is lost at that point (either due to an opportunity attack/readied action or the casting of another concentration spell) is that character permitted to continue moving the remainder of their dash distance?