Simon Peyton-Jones wrote:

Of course -fallow-undecidable-instances still lifts all restrictions,
and then all bets are off.

Is the behaviour of GHC with -fallow-undecidable-instances (and -fcontext-stack) well-understood and specifiable? It is a very useful option, as you can join (meet?) classes like this:

  class P a
  class Q a

  class (P a,Q a) => PQ a
  instance (P a,Q a) => PQ a

--
Ashley Yakeley

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