[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             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:                    |
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Comment (by RyanGlScott):

 goldfire, are you saying this code should emit a warning? If so, what can
 be done to fix the code?

 FWIW, this bug also prevents the
 [https://github.com/ekmett/constraints/tree/1649e382b8dffe5e2694d0959caeba65e87a411e
 constraints] library from building with GHC HEAD, although the bug appears
 in a slightly different form. Here's a stripped-down example that compiles
 with GHC 8.0.1, but not GHC HEAD:

 {{{#!hs
 {-# LANGUAGE ConstraintKinds #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE RankNTypes #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 module ConstraintsBug where

 import GHC.TypeLits (Nat)
 import Unsafe.Coerce (unsafeCoerce)

 data Dict a where
   Dict :: a => Dict a

 newtype a :- b = Sub (a => Dict b)

 axiom :: forall a b. Dict (a ~ b)
 axiom = unsafeCoerce (Dict :: Dict (a ~ a))

 type Divides n m = n ~ Gcd n m
 type family Gcd :: Nat -> Nat -> Nat where

 dividesGcd :: forall a b c. (Divides a b, Divides a c) :- Divides a (Gcd b
 c)
 dividesGcd = Sub axiom
 }}}

 {{{
 $ /opt/ghc/head/bin/ghc ConstraintsBug.hs
 [1 of 1] Compiling ConstraintsBug   ( ConstraintsBug.hs, ConstraintsBug.o
 )

 ConstraintsBug.hs:26:14: error:
     • Couldn't match type ‘a’ with ‘Gcd a b’
       ‘a’ is a rigid type variable bound by
         the type signature for:
           dividesGcd :: forall (a :: Nat) (b :: Nat) (c :: Nat).
                         (Divides a b, Divides a c) :- Divides a (Gcd b c)
         at ConstraintsBug.hs:25:1-77
       Inaccessible code in
         a type expected by the context:
           (Divides a b, Divides a c) => Dict a ~ Gcd a (Gcd b c)
     • In the first argument of ‘Sub’, namely ‘axiom’
       In the expression: Sub axiom
       In an equation for ‘dividesGcd’: dividesGcd = Sub axiom
     • Relevant bindings include
         dividesGcd :: (Divides a b, Divides a c) :- Divides a (Gcd b c)
           (bound at ConstraintsBug.hs:26:1)
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:4>
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