What happens to a character's current HP when their hit point maximum is reduced?

Question

Suppose a situation in which my character is currently at 10 HP out of a maximum of 20 HP. If my maximum HP is reduced by 10, do I also lose the 10 current HP I have?

Situation 1. The answer found here - What happens to the current hit points when Aid ends? - seems to indicate that my current HP would remain at 10 (keep in mind I want a general answer, not specifically for Aid).

Situation 2. When I read the PHB page 177 concerning Hit Points, I see that:

If your Constitution modifier changes, your hit point maximum changes as well, as though you had the new modifier from 1st level.

Which implies to me that if my hit point maximum were to change due to my Constitution score being reduced, then I have effectively taken 10 damage as if my hit points were actually 10 from 1st level (like a ret-con).

If my reading of Situation 2 is correct and that is how it works upon Con score reduction, then would this situation apply for any loss of maximum HP, such as with a Wraith?

Answer 1

Your current hit points may never exceed your hit point maximum. Unless a rule specifically states it also affects hit points (not just hit point maximum), only the hit point maximum is lowered.

If your hit point maximum drops to below your current hit points, your current hit points will be reduced to the new hit point maximum.

Keep in mind that certain effects cause both your hit points and hit point maximum to be reduced. An example is the attack of a Wraith which causes damage and then lowers the characters hit point maximum if the save is failed.

Example:

A level 3 Barbarian with 14 Constitution and 39 hit point maximum with 31 current hit points loses 1 constitution due to disease. His hit point maximum drops to 36, but his current hit points stay at 31.

The disease is horrible, and even before healing he loses another 4 constitution (total of 9 now). His hit point maximum drops to 30, and, since his hit point maximum is now lower than his current hit points, his current hit points drop to 30 as well.