Typechecker plugin proposal, ticket #15147

[Nicolas Frisby]

If you know of any typechecker plugin authors I've missed, please add them
to the thread.

In the comments of ticket https://gitlab.haskell.org/ghc/ghc/issues/15147
several of us agreed that the behavior of the typechecker plugin interface
should change: GHC should no longer unflatten the fmvs in the Wanteds
before passing them to the plugin.

This is presumably a breaking change to plugins. We might be able to get by
with some sort of flag indicating if the plugin expects the Wanteds
flattened or not, but ideally GHC would just always pass them flattened.
I'm unaware of any established policy about interface changes at this
level, whether we've somehow committed to backwards-compatibility here or
not. Anyone know?

Plugin authors: would you look over the ticket comments and share your
thoughts here? We're looking to build some sort of consensus about how to
proceed without shocking the API users.

Thank you for your time. -Nick
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