[GHC] #12557: Regression in type inference with ImpredicativeTypes

[GHC]

#12557: Regression in type inference with ImpredicativeTypes
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        Reporter:  slindley          |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  invalid           |  ImpredicativeTypes
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

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


Comment:

 This is a bug in 7.10.

 Basically, a lambda-bound variable like `\g` above can never get a higher
 rank type (one involving a forall), unless there is an enclosing type
 signature so that it's clear ''at the binding site'' what its type should
 be.

 I don't know any way to fix this.  Do not rely on `-XImpredicativeTypes`;
 it is an incomplete, inconsistent and generally broken feature that I
 should probably remove.  I just don't know how to do a better job, sorry!

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12557#comment:2>
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