[ghc-steering-committee] Proposal #612: Change semantics of -fhpc to not accumulate data over multiple runs, rec: accept

[Moritz Angermann]

Dear Committee members,

In Proposal #612 <https://github.com/ghc-proposals/ghc-proposals/pull/612>,
David Binder suggests changing the semantics from `-fhpc` to auto-ingest
existing .tix files for accumulation, towards _not_ auto-ingesting them and
instead overwriting
the .tix file with the coverage of the latest run.

I agree with his assessment that the current behaviour easily leads to
unexpected surprises. He suggests adding a flag --read-tix-file= to control
this behaviour and defaulting that to _no_, with a grace and
deprecation period prior informing the user that the currently accumulating
feature will change in a future GHC release.

Adding the flag to control this behaviour is fairly uncontroversial I hope,
however I'd like you to. Weight in on the default. Should we change the
default behaviour, or only add the flag?

I'd recommend changing the default to --read-tix-file=no after a
deprecation period.

Best,
 Moritz
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