[GHC] #13713: fdefer-type-errors makes missing import errors disappear

17 May 2017

      #13713: fdefer-type-errors makes missing import errors disappear
-------------------------------------+-------------------------------------
           Reporter:  nh2            |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #12529
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 I have this code

 {{{#!hs
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeApplications #-}

 import           Data.Type.Equality ((:~:)(Refl))
 import           GHC.TypeLits
 import qualified Numeric.LinearAlgebra as LA
 import           Numeric.LinearAlgebra.Static

 toVec :: forall n . (KnownNat n) => LA.Vector Double -> R n
 toVec vec = withVector vec $ \(v :: R n2) -> case sameNat (Proxy @n)
 (Proxy @n2) of
   Just Refl -> v
   Nothing -> error "wrong dimensions"
 }}}

 Notably I forgot to import `Proxy`.

 Without `-fdefer-type-errors` I get this error:

 {{{
 ➤ nix-shell -p "haskellPackages.ghcWithPackages (pkgs:[pkgs.hmatrix])"
 --pure --run 'ghci ghc-8.0.2-proxy-confusing-error.hs'
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( ghc-8.0.2-proxy-confusing-error.hs,
 interpreted )

 ghc-8.0.2-proxy-confusing-error.hs:12:60: error:
     Data constructor not in scope: Proxy

 ghc-8.0.2-proxy-confusing-error.hs:12:60: error:
     * Cannot apply expression of type `t1'
       to a visible type argument `n'
     * In the first argument of `sameNat', namely `(Proxy @n)'
       In the expression: sameNat (Proxy @n) (Proxy @n2)
       In the expression:
         case sameNat (Proxy @n) (Proxy @n2) of {
           Just Refl -> v
           Nothing -> error "wrong dimensions" }
 Failed, modules loaded: none.
 }}}

 but with `-fdefer-type-errors` the `Data constructor not in scope: Proxy`
 is gone!

 {{{
 ➤ nix-shell -p "haskellPackages.ghcWithPackages (pkgs:[pkgs.hmatrix])"
 --pure --run 'ghci ghc-8.0.2-proxy-confusing-error.hs -fdefer-type-errors'
 GHCi, version 8.0.2: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( ghc-8.0.2-proxy-confusing-error.hs,
 interpreted )

 ghc-8.0.2-proxy-confusing-error.hs:12:60: error:
     * Cannot apply expression of type `t1'
       to a visible type argument `n'
     * In the first argument of `sameNat', namely `(Proxy @n)'
       In the expression: sameNat (Proxy @n) (Proxy @n2)
       In the expression:
         case sameNat (Proxy @n) (Proxy @n2) of {
           Just Refl -> v
           Nothing -> error "wrong dimensions" }
 Failed, modules loaded: none.
 }}}

 This is probably related to #12529.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13713
GHC http://www.haskell.org/ghc/
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