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

2 files changed, 16 insertions, 0 deletions

diff --git a/testsuite/tests/deSugar/should_compile/T14547.hs b/testsuite/tests/deSugar/should_compile/T14547.hs
new file mode 100644
index 0000000000..02ff2e618c
--- /dev/null
+++ b/testsuite/tests/deSugar/should_compile/T14547.hs
@@ -0,0 +1,15 @@
+{-# 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
diff --git a/testsuite/tests/deSugar/should_compile/all.T b/testsuite/tests/deSugar/should_compile/all.T
index 2d361464a6..14140737d8 100644
--- a/testsuite/tests/deSugar/should_compile/all.T
+++ b/testsuite/tests/deSugar/should_compile/all.T
@@ -100,6 +100,7 @@ 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'])