Allow contrib to alter coupon availability check

I stumbled upon this issue when researching the ability to limit coupon usage to specific user (order owner). The business case is - send 10% off unique coupon code to the user upon their birthday. Coupon is only valid for 1 month and can only be redeemed by the same user who received the coupon. Other users, authenticated or anonymous, can't use that particular coupon (but they may have their own coupons).

For the performance reason promotion per user is not sustainable. All that's left is the ability to connect coupon with specific user and limit the availability.

We have this requirement, too.
In our B2B use-case, multiple registered users belong to the same "customers". So we need to summarize the redemptions of all users of the same "customer". That could easily be done by altering the result of the available() function.
I suggest to introduce an event here.

I agree with Matt here, let's not bloat coupons... The promotion is where you're supposed to set the conditions. Adding an event might seem harmless, but guessing this still has a performance footprint...
Additionally, if you're comfortable writing an event subscriber... Then swapping the coupon entity class and overridding the available() method isn't much different.

Additionally, if you're comfortable writing an event subscriber... Then swapping the coupon entity class and overridding the available() method isn't much different.

We did it like that ... first.
Now we found a much easier solution by swapping the PromotionUsage service against a class that overwrites load().

The only issue is that it is not an API and it takes time to read the code to find a solution.