[GHC] #8268: Local annotations ignored in ambiguity check

[GHC]

#8268: Local annotations ignored in ambiguity check
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        Reporter:  maxs                       |            Owner:
            Type:  bug                        |           Status:  closed
        Priority:  highest                    |        Milestone:
       Component:  Compiler (Type checker)    |          Version:  7.7
      Resolution:  invalid                    |         Keywords:
Operating System:  Unknown/Multiple           |     Architecture:
 Type of failure:  GHC rejects valid program  |  Unknown/Multiple
       Test Case:                             |       Difficulty:  Unknown
        Blocking:                             |       Blocked By:
                                              |  Related Tickets:
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Changes (by simonpj):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 I'm afraid the error is quite right: `fst`'s type is ambiguous.  Suppose
 we accepted its type. Then we said:
 {{{
 fst2 :: forall f a b. Unlift f (f a, f b) => f (Plain (f a), Plain (f b))
 -> f a
 fst2 = fst
 }}}
 The type sig for `fst2` is identical to `fst`, and `fst2 = fst`, so
 obviously it should typecheck.

 But it won't for the reason specified: there is no way to fix `b` in the
 call to `fst` inside `fst2`, because it only appears under a type
 function. In a more general context you'll find `fst` very hard to call.

 Does that make sense?

 Simon

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8268#comment:1>
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