Revert "Normalize the element type of ListPat, fix #14547"

This reverts commit 361d23a8.

compiler/deSugar/Check.hs

@@ -790,13 +790,10 @@ 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` norm_e_ty ->
+    , norm_elem_ty `eqType` 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'

testsuite/tests/deSugar/should_compile/T14547.hs

-{-# LANGUAGE OverloadedLists #-}
-{-# LANGUAGE TypeFamilies #-}
-
-module T14547 where
-
-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

testsuite/tests/deSugar/should_compile/all.T

@@ -100,7 +100,6 @@ test('T13290', normal, compile, [''])
 test('T13257', normal, compile, [''])
 test('T13870', normal, compile, [''])
 test('T14135', normal, compile, [''])
-test('T14547', normal, compile, ['-Wincomplete-patterns'])
 test('T14773a', normal, compile, ['-Wincomplete-patterns'])
 test('T14773b', normal, compile, ['-Wincomplete-patterns'])
 test('T14815', [], run_command, ['$MAKE -s --no-print-directory T14815'])