osu-lazer

Make Difficulty Adjust and Target Practice mods incompatible (#36524 )

Closes https://github.com/ppy/osu/issues/36490.

While I'm out here already taking heat for deleting mod combinations let
me do more of that.

The main problem with the mod combination here, again, is that the
application of the mods does not commute.

- The current behaviour is that TP is applied first, then DA. This is,
again, "enforced" by the mod select overlay implicitly enforcing order
of the mod instances in the global mod bindable to match the display
order of mods inside it.

Even this doesn't "work" correctly as is, because as the bug reporter
points out, if they throw on DA with no changes expecting the map's
default AR to be applied, it still gets halved. This is because DA works
in a way wherein if you don't touch the AR slider, DA does not touch AR.
Which means that the DA slider should *really* be at *half* of the map's
base AR by default in this case because TP is active. How do you program
this?

- The *alternative* behaviour would be that DA is applied first, then
TP. This in turn would mean that the effective range of AR adjustment
offered by DA when TP is active would be halved to [0, 5] (or [-5, 5.5]
with extended ranges). How do you program this?

The above is just client-side concerns, while leaving out the other
giant concern, which is "how do you get every single place in the game
that may want to apply mods to a beatmap to apply them *in the same
order*?". Then extend that to server-side components, then extend that
to every external library that may want to re-implement SR/PP
calculations, etc. etc.

One additional remark:
What the bug reporter *did not* say however, but I am saying, is that
there's an elephant in the room, and that is the Easy mod, which *also*
changes AR, but also *happens* to apply commutatively with Target
Practice simply because both mods are implemented to halve the AR, which
means that the order of application doesn't matter. If I were *really*
bent on being a bad guy and just deleting mod combinations
indiscriminately, I'd delete that one as well. But I'm not doing that.