fdefer-type-errors makes missing import errors disappear

I have this code

{-# 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 (closed).

Trac metadata

Trac field	Value
Version	8.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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CC	nh2
Operating system
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