Add test for updating a record with existentially quantified fields.

Test Plan: ./validate

Reviewers: austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1193

testsuite/tests/typecheck/should_compile/all.T

@@ -469,3 +469,4 @@ test('T10564', normal, compile, [''])
 test('T10632', normal, compile, [''])
 test('T10642', normal, compile, [''])
 test('T10744', normal, compile, [''])
+test('update-existential', normal, compile, [''])

testsuite/tests/typecheck/should_compile/update-existential.hs

+{-# LANGUAGE
+            NoImplicitPrelude
+           , ExistentialQuantification
+  #-}
+
+module Test where
+
+hGetContents handle_ = handle_{ haType=SemiClosedHandle}
+
+data HandleType = SemiClosedHandle
+
+class Show a where
+  show :: a -> a
+
+-- they have to check whether the handle has indeed been closed.
+data Handle__
+  = forall dev . (Show dev) =>
+    Handle__ {
+      haDevice      :: !dev,
+      haType        :: HandleType           -- type (read/write/append etc.)
+}