Normalize the element type of ListPat, fix #14547

The element type of `List` maybe a type family instacen, rather than a
trivial type.  For example in Trac #14547,

```
{-# LANGUAGE TypeFamilies, OverloadedLists #-}

class Foo f where
        type It f
        foo :: [It f] -> f

data List a = Empty | a :! List a deriving Show

instance Foo (List a) where
        type It (List a) = a
        foo [] = Empty
        foo (x : xs) = x :! foo xs
```

Here the element type of `[]` is `It (List a)`, we should also normalize
it as `a`.

Test Plan: make test TEST="T14547"

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #14547

Differential Revision: https://phabricator.haskell.org/D4624

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler/deSugar/Check.hs b/compiler/deSugar/Check.hs
index 545aacef51..7a5f889423 100644
--- a/compiler/deSugar/Check.hs
+++ b/compiler/deSugar/Check.hs
@@ -790,10 +790,13 @@ translatePat fam_insts pat = case pat of
   -- overloaded list
   ListPat (ListPatTc elem_ty (Just (pat_ty, _to_list))) lpats
     | Just e_ty <- splitListTyConApp_maybe pat_ty
+    , (_, norm_e_ty) <- normaliseType fam_insts Nominal e_ty
+         -- e_ty can be a type family instance, like
+         -- `It (List a)`, but we prefer `a`, see Trac #14547
     , (_, norm_elem_ty) <- normaliseType fam_insts Nominal elem_ty
          -- elem_ty is frequently something like
          -- `Item [Int]`, but we prefer `Int`
-    , norm_elem_ty `eqType` e_ty ->
+    , norm_elem_ty `eqType` norm_e_ty ->
         -- We have to ensure that the element types are exactly the same.
         -- Otherwise, one may give an instance IsList [Int] (more specific than
         -- the default IsList [a]) with a different implementation for `toList'