[GHC] #13264: GHC panic with (->) generalization branch while compiling lens

[GHC]

#13264: GHC panic with (->) generalization branch while compiling lens
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        Reporter:  bgamari           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 Maybe, but not necessarily. After all, the change we're proposing replaces
 a panic with a `Nothing`. In theory, no code (except the case we've
 identified in `can_eq_nc'`) should evaluate to the panic, so changing
 behavior there should have no effect beyond `can_eq_nc'`.

 However, practice does not always equate to theory, and this choice might
 change a helpful panic into very obscure behavior. So if the function
 ''is'' widely used, then I learn more toward the new-ADT approach: define
 something like

 {{{#!hs
 data SplitResult a b = Split a b | NotSplittable | NoRuntimeRep
 }}}

 where `NotSplittable` means that the type is a `FunTy` that would
 ordinarily be splittable, but retrieving the `RuntimeRep` failed. This
 failure should happen only when a) something is ill-kinded, or b)
 something needs a good zonk. This new ADT could be used in both
 `splitTyConApp` and `splitAppTy`.

 Or here's another approach: you're calling `tcRepSplitTyConApp_maybe`.
 Note the `tc`. How would we fare if this function were put into the `TcM`
 monad? Then, if the `RuntimeRep` were unavailable, we could zonk on the
 fly and succeed.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13264#comment:7>
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