[GHC] #9607: Type checking regression between GHC 7.6 and 7.8

[GHC]

#9607: Type checking regression between GHC 7.6 and 7.8
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              Reporter:  jstolarek   |            Owner:
                  Type:  bug         |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:  invalid     |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  GHC         |  Related Tickets:
  rejects valid program              |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by jmccarty):

 Indeed, I'm guessing it was a bug that 7.6 accepted my program. I would
 argue that the type is not per se ambiguous, GHC just can't see it.
 `leftUnit` is precisely as ambiguous as `rightUnit`, but `leftUnit` is
 considered unambiguous (I think) because the type family application
 simplifies, making the signature `Tensor s -> Tensor (n ': n ': s)`, and I
 guess `':` is accepted as injective.

 In principle, if the argument and return types of rightUnit are
 instantiated to some type, then `n` can be instantiated to a unique type
 (but GHC would have to invert `(s ++)` to determine it!).

 I don't really understand the ambiguity check, but I think GHC wants to
 check that `(s ++ '[n0, n0]) ~ (s ++ '[n, n])` implies `n0 ~ n`? This is
 provably true, but I certainly don't expect GHC to construct the inductive
 proof of this fact.

 I was hoping the type family injectivity proposal would allow declaring
 such a fact (but again, I don't know that GHC could verify it).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9607#comment:3>
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