New restrictions to open type families

[Akio Takano]

Hi,

Looking at the ticket [1] and a draft paper linked there [2] , I learned
that in GHC 7.8, two type family instances are considered overlapping if
the argument lists unify in the presence of infinite types.

My question is, why is this restriction necessary? A footnote in the paper
states that it was not possible with GHC 7.6 to actually exploit the
inconsistency. Would it be difficult to solve the problem by making this
behaviour part of the specification, rather than accepting fewer programs?

[1] http://ghc.haskell.org/trac/ghc/ticket/8154
[2] http://www.cis.upenn.edu/~eir/papers/2014/axioms/axioms-extended.pdf

Regards,
Takano Akio
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