If a half-feat were to be taken that would give an increase to an ability score already at 20, could the "increase" be used for a different score?

Question

Say I had a character with an Intelligence score of 20, perhaps acquired by a racial bonus or previous ASIs, then they took, say, the Keen Mind feat.

Now, in addition to everything else Keen Mind grants, it also grants an increase to your Intelligence score by one, but that increase can't take the score past 20.

Now, my question is, if I wanted to have the extra benefits of Keen Mind, but I already have an Intelligence of 20, is there any way to keep the benefit of the half-ASI it gives, or is the +1 just wasted?

Answer 1

Not unless the feat says it can.

If a feat increases your Intelligence score by 1, for instance, and your Intelligence score is already at 20 (and your maximum is unchanged at 20 as well), there is no rule allowing you to pick a different ability score to increase instead.

There are some "half-feats" that give you a number of different options to choose from. For instance, the Elven Accuracy feat (Xanathar's Guide to Everything, p. 74) lets you increase either your Dexterity, Intelligence, Wisdom, or Charisma score by 1; Resilient (PHB, p. 168) lets you increase any one ability score by 1 and gain proficiency in the corresponding saving throw. Otherwise, you are limited to the specific ability score that the feat says you can increase.

(As always, a DM can house-rule otherwise; if they're considering doing so, they should of course be aware that this increases the usefulness/versatility of such feats.)