Pattern synonyms and higher rank types

This patch fixes two separate bugs which contributed to making
Trac #13752 go wrong

1.  We need to use tcSubType, not tcUnify,
    in tcCheckPatSynDecl.tc_arg.

    Reason: Note [Pattern synonyms and higher rank types]

2.  TcUnify.tc_sub_type had a special case designed to improve error
    messages; see Note [Don't skolemise unnecessarily].  But the
    special case was too liberal, and ended up using unification
    (which led to rejecting the program) when it should instead taken
    the normal path (which accepts the program).

    I fixed this by making the test more conservative.

3 files changed, 19 insertions, 0 deletions

diff --git a/testsuite/tests/patsyn/should_compile/T13752.hs b/testsuite/tests/patsyn/should_compile/T13752.hs
new file mode 100644
index 0000000000..f9ff606ee7
--- /dev/null
+++ b/testsuite/tests/patsyn/should_compile/T13752.hs
@@ -0,0 +1,9 @@
+{-# LANGUAGE TypeFamilies, ConstraintKinds, PatternSynonyms, RankNTypes #-}
+
+module T13752 where
+
+newtype Arrange = Arrange {getArrange :: [Int] -> [Int]}
+
+pattern Heh :: (c ~ ((~) Int)) => (forall a. c a => [a] -> [a]) -> Arrange
+-- pattern Heh :: (forall a. (Int ~ a) => [a] -> [a]) -> Arrange
+pattern Heh f <- Arrange f
diff --git a/testsuite/tests/patsyn/should_compile/T13752a.hs b/testsuite/tests/patsyn/should_compile/T13752a.hs
new file mode 100644
index 0000000000..2c417f9fed
--- /dev/null
+++ b/testsuite/tests/patsyn/should_compile/T13752a.hs
@@ -0,0 +1,8 @@
+{-# LANGUAGE PatternSynonyms, RankNTypes #-}
+
+module T13752a where
+
+data T = MkT (forall a. a->a)
+
+pattern P :: (Int -> Int) -> T
+pattern P x <- MkT x
diff --git a/testsuite/tests/patsyn/should_compile/all.T b/testsuite/tests/patsyn/should_compile/all.T
index fa8a3d8455..30319c7050 100644
--- a/testsuite/tests/patsyn/should_compile/all.T
+++ b/testsuite/tests/patsyn/should_compile/all.T
@@ -68,3 +68,5 @@ test('T13441', normal, compile, [''])
 test('T13441a', normal, compile, [''])
 test('T13441b', normal, compile_fail, [''])
 test('T13454', normal, compile, [''])
+test('T13752', normal, compile, [''])
+test('T13752a', normal, compile, [''])