As of the 2018 PHB errata, the creature generated by Simulacrum is a construct. Constructs cannot be healed by many spells or features, such as Cure Wounds or a Paladin's Lay on Hands (that specifically state that their healing cannot be applied to Constructs/Undead). As the Simulacrum spell lays out a costly HP-restoration method for it "using rare herbs and minerals worth 100 gp per hit point it regains", it seems that the RAI (Rules As Intended) was for that method to be the only viable way to regain hit points.
This leads into two questions:
- Fighter has the Second Wind feature, that allows it to use a Bonus Action to "regain hit points equal to 1d10 + your fighter level.". Since it's using the same 'regain' word, would this Bonus Action allow a Construct-type creature with the appropriate levels and/or a Simulacrum to regain hit points?
This one is a little iffy in my opinion, as the 'regain' word is specifically called out in the text of Simulacrum.
- Spells such as Aid and Heroes' Feast exist, that respectively have the text of "Each target's hit point maximum and current hit points increase by 5 for the duration" and "Its hit point maximum also increases by 2d10, and it gains the same number of hit points". Since both of these spells "increase" HP or cause the creature to "gain" HP, do those work on Construct-type creatures and/or on a Simulacrum?
This one seems more clear-cut. Increasing HP / Gaining HP isn't specifically "healing" damage or "regaining" damage. When Aid or Heroes' Feast expire, they only reduce current hit points if they are above your maximum hit points, so I'd understand it that a cast of a 2nd-level Aid on a Construct/Simulacrum with... say, 10/40 HP, would increase that to 15/45 HP, and upon expiry of Aid would then be at 15/40 HP.
Before I posted, I did find 'similar' questions, but they seem to focus on "Healing", or be treating the HP of Aid as separate/"healable" HP.