[GHC] #10715: Possible regression in Coercible a (X a) between 7.8 and 7.10

[GHC]

#10715: Possible regression in Coercible a (X a) between 7.8 and 7.10
-------------------------------------+-------------------------------------
              Reporter:  inaki       |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 In upgrading to7.10, code of the form
 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 import Data.Coerce (coerce, Coercible)

 data X a

 doCoerce :: Coercible a (X a) => a -> X a
 doCoerce = coerce
 }}}
 fails to compile in 7.10.1 and 7.10.2 with the error
 {{{
 testCoerce.hs:6:13:
     Could not deduce (a ~ X a)
     from the context (Coercible a (X a))
       bound by the type signature for
                  doCoerce :: Coercible a (X a) => a -> X a
       at testCoerce.hs:6:13-41
       ‘a’ is a rigid type variable bound by
           the type signature for doCoerce :: Coercible a (X a) => a -> X a
           at testCoerce.hs:6:13
     Relevant role signatures: type role X phantom
     In the ambiguity check for the type signature for ‘doCoerce’:
       doCoerce :: forall a. Coercible a (X a) => a -> X a
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘doCoerce’:
       doCoerce :: Coercible a (X a) => a -> X a
 }}}
 while it works in 7.8.4.

 Surprisingly (to me at least), the code works in 7.10.1 and 7.10.2 if I
 change it to
 {{{#!hs
 {-# LANGUAGE FlexibleContexts #-}
 import Data.Coerce (coerce, Coercible)

 data X a

 doCoerce :: Coercible a (X b) => a -> X a
 doCoerce = coerce
 }}}
 while it fails to compile in 7.8.4 with the error
 {{{
 testCoerce.hs:6:13:
     Could not coerce from ‘a’ to ‘X b0’
       because ‘a’ and ‘X b0’ are different types.
       arising from the ambiguity check for ‘doCoerce’
     from the context (Coercible a (X b))
       bound by the type signature for
                  doCoerce :: Coercible a (X b) => a -> X a
       at testCoerce.hs:6:13-41
     The type variable ‘b0’ is ambiguous
     In the ambiguity check for:
       forall a b. Coercible a (X b) => a -> X a
     To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
     In the type signature for ‘doCoerce’:
       doCoerce :: Coercible a (X b) => a -> X a
 }}}

 The coercion pattern may look a bit funny, but it is rather useful when
 one has newtypes of the form
 {{{#!hs
 newtype Y = Y (ForeignPtr Y)
 }}}
 which appear naturally when writing bindings to C libraries, and one wants
 to get access to the underlying ForeignPtr from Y (here X -> ForeignPtr).
 The relevant Coercible instance here is Coercible Y (ForeignPtr Y), as
 above.

 I would have expected the version with context "Coercible a (X a)" to be
 accepted, as 7.8.4 does, since it seems to be a specialization of the more
 general coerce, but maybe I am missing something?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10715>
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