Here's my rationale for a deprecation period.
Someone who uses hpc, might rely on the accumulating behaviour. They need a way to be informed about the upcoming change, otherwise they won't know that they need to pass a new flag going forward (and they can
even start doing that at that point to be defensive). I also hope the deprecation note would show up in automated tooling, which can then be proactively adjusted.
To me the compiler is _the_ interface to the end user. Chances that someone reads the proposals, GHC changelog are very low. Chances someone reads the user guide in detail for each GHC release are low as well,
only after the fact, if someone broke will someone start trying to figure out why something broke. At that point, I think we've lost. They are frustrated already that something they knew worked just broke.
So to me the deprecation note is basically the signage towards the end user that uses hpc, that they will need to adjust their ghc invocation if they rely on the current behaviour, and also a courtesy heads up that the behaviour
they might rely on changes, _and_ how to adjust and get their old behaviour (they may need/rely on) back.
To address your two points in detail:
> Is a deprecation period necessary? Or even useful (how do we signal the upcoming change in default, and how do we expect people would act on it)?
- As the flag currently does not exist, call GHC with hpc without the new flag will issue a warning. (e.g. current users will be informed).
- The warning message will let the user know what to pass to ghc to retain the old (current) behaviour, or opt in to the new (future current) behaviour, depending on their needs.
If we did neither, the user would at some point just notice an existing workflow magically stopped or in this case even worse, the silent aggregation of .tix data would stop, and they
might _assume_ the data was the aggregate but it wasn't. A silent change in default behaviour would be rather painful to notice, debug, and rectify for users. At least that's my expectation.
(And I know I personally would be furious if a compiler pulled that on me).
Best,
Moritz