[GHC] #14323: Occurs check regressions in GHC 8.2.1 (and HEAD)

[GHC]

#14323: Occurs check regressions in GHC 8.2.1 (and HEAD)
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Inspired by adamse's comment at
 https://ghc.haskell.org/trac/ghc/ticket/14317#comment:3, I've discovered
 some programs which behave differently on GHC 8.0.2, 8.2.1, and HEAD.

 -----

 First, there's this program:

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 hm1 :: b ~ f b => b -> f b
 hm1 x = x
 }}}

 On GHC 8.0.2, this compiles (with a warning):

 {{{
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:4:1: warning: [-Woverlapping-patterns]
     Pattern match is redundant
     In an equation for ‘hm1’: hm1 x = ...
 }}}

 But on GHC 8.2.1, it errors:

 {{{
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:4:9: error:
     • Could not deduce: b ~ f b
       from the context: b ~ f b
         bound by the type signature for:
                    hm1 :: forall b (f :: * -> *). b ~ f b => b -> f b
         at Bug.hs:3:1-26
       ‘b’ is a rigid type variable bound by
         the type signature for:
           hm1 :: forall b (f :: * -> *). b ~ f b => b -> f b
         at Bug.hs:3:1-26
     • In the expression: x
       In an equation for ‘hm1’: hm1 x = x
     • Relevant bindings include
         x :: b (bound at Bug.hs:4:5)
         hm1 :: b -> f b (bound at Bug.hs:4:1)
   |
 4 | hm1 x = x
   |         ^
 }}}

 And on GHC HEAD, it fails with a different error!

 {{{
 GHCi, version 8.3.20171004: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 [1 of 1] Compiling Main             ( Bug.hs, interpreted )

 Bug.hs:4:9: error:
     • Occurs check: cannot construct the infinite type: b ~ f b
     • In the expression: x
       In an equation for ‘hm1’: hm1 x = x
     • Relevant bindings include
         x :: b (bound at Bug.hs:4:5)
         hm1 :: b -> f b (bound at Bug.hs:4:1)
   |
 4 | hm1 x = x
   |         ^
 }}}

 The same errors occur if you give `hm1` the type `hm1 :: b ~ f b => f b ->
 b`.

 -----

 What happens if you try `Coercible` instead of `(~)`? Consider this
 variant of the program above:

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 import Data.Coerce

 hm2 :: Coercible b (f b) => b -> f b
 hm2 = coerce
 }}}

 On GHC 8.0.2, 8.2.1, and HEAD, this compiles (without warnings). Good. But
 if you change the type to use the symmetric version of that constraint:

 {{{#!hs
 {-# LANGUAGE GADTs #-}

 import Data.Coerce

 hm3 :: Coercible (f b) b => b -> f b
 hm3 = coerce
 }}}

 Then on GHC 8.0.2, it compiles without warnings, but on 8.2.1 and HEAD it
 fails!

 Here is the error with 8.2.1:

 {{{
 Bug3.hs:6:7: error:
     • Could not deduce: Coercible b (f b)
         arising from a use of ‘coerce’
       from the context: Coercible (f b) b
         bound by the type signature for:
                    hm3 :: forall (f :: * -> *) b. Coercible (f b) b => b
 -> f b
         at Bug3.hs:5:1-36
       ‘b’ is a rigid type variable bound by
         the type signature for:
           hm3 :: forall (f :: * -> *) b. Coercible (f b) b => b -> f b
         at Bug3.hs:5:1-36
     • In the expression: coerce
       In an equation for ‘hm3’: hm3 = coerce
     • Relevant bindings include hm3 :: b -> f b (bound at Bug3.hs:6:1)
   |
 6 | hm3 = coerce
   |       ^^^^^^
 }}}

 And on HEAD:

 {{{
 Bug3.hs:6:7: error:
     • Occurs check: cannot construct the infinite type: b ~ f b
         arising from a use of ‘coerce’
     • In the expression: coerce
       In an equation for ‘hm3’: hm3 = coerce
     • Relevant bindings include hm3 :: b -> f b (bound at Bug3.hs:6:1)
   |
 6 | hm3 = coerce
   |       ^^^^^^
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14323>
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