[GHC] #11948: GHC forgets constraints

[GHC]

#11948: GHC forgets constraints
-------------------------------------+-------------------------------------
           Reporter:  crockeea       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
           Keywords:                 |  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:
-------------------------------------+-------------------------------------
 Possibly related: #10338

 The following program should compile, but fails with the error:

 {{{
 Could not deduce (Bar (F zq) zq) arising from a use of ‘bar’
     from the context (Bar (Foo (F zq)) (Foo zq))
       bound by the type signature for
                  bug :: Bar (Foo (F zq)) (Foo zq) => Foo (F zq) -> Foo zq
 }}}

 This is definitely incorrect: I am providing a `Bar` constraint in the
 context of `bug`, but GHC is asking for constraints so that it can resolve
 to the instance declared in Bar.hs.

 A few workarounds I've found so far, which may or may not help find the
 bug:
 1. Adding `-XTypeFamilies` to Main.hs makes the program compile.
 2. ''Removing'' the type signature from `x` in `bug` makes the program
 compile.
 3. Defining `bug` without a `let` as `bug sk = bar sk :: Foo zq` or `bug
 sk = bar sk` makes the program compile.

 Main.hs:

 {{{
 {-# LANGUAGE FlexibleContexts, ScopedTypeVariables #-}

 import Bar

 bug :: forall zq .
   (Bar (Foo (F zq)) (Foo zq))
   => Foo (F zq) -> Foo zq
 bug sk =
   let x = bar sk :: Foo zq
   in x
 }}}

 Bar.hs

 {{{
 {-# LANGUAGE MultiParamTypeClasses, TypeFamilies #-}

 module Bar where

 type family F b

 newtype Foo r = Foo r

 type instance F (Foo r) = Foo (F r)

 class Bar a b where
   bar :: a -> b

 instance (Bar a b) => Bar (Foo a) (Foo b)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11948>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler