[commit: ghc] master: Add (failing) test case for #11347 (1a8b752)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/1a8b752d8b03266aca3e83f79c311056d6c43e00/ghc

>---------------------------------------------------------------

commit 1a8b752d8b03266aca3e83f79c311056d6c43e00
Author: Joachim Breitner <mail at joachim-breitner.de>
Date:   Tue Jan 5 10:56:47 2016 +0100

    Add (failing) test case for #11347
    
    Unfortunately, I could not add the expected error message, so if someone
    accidentally fixes this bug, this test will still be failing (no harm).
    But maybe someone stumbles over it then and can update the expected
    output.


>---------------------------------------------------------------

1a8b752d8b03266aca3e83f79c311056d6c43e00
 testsuite/tests/typecheck/should_fail/T11347.hs     | 19 +++++++++++++++++++
 testsuite/tests/typecheck/should_fail/T11347.stderr |  2 ++
 testsuite/tests/typecheck/should_fail/all.T         |  1 +
 3 files changed, 22 insertions(+)

diff --git a/testsuite/tests/typecheck/should_fail/T11347.hs b/testsuite/tests/typecheck/should_fail/T11347.hs
new file mode 100644
index 0000000..eaffdfd
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T11347.hs
@@ -0,0 +1,19 @@
+-- Should AllowAmbiguousTypes relaly be needed here?
+{-# LANGUAGE TypeFamilies, GeneralizedNewtypeDeriving, MultiParamTypeClasses, FlexibleInstances, AllowAmbiguousTypes #-}
+module T11347 where
+
+newtype Id1 a = MkId1 a
+newtype Id2 a = MkId2 (Id1 a) deriving (UnsafeCast b)
+
+type family Discern a b
+type instance Discern (Id1 a) b = a
+type instance Discern (Id2 a) b = b
+
+class UnsafeCast to from where
+  unsafe :: from -> Discern from to
+
+instance UnsafeCast b (Id1 a) where
+  unsafe (MkId1 x) = x
+
+unsafeCoerce :: a -> b
+unsafeCoerce x = unsafe (MkId2 (MkId1 x))
diff --git a/testsuite/tests/typecheck/should_fail/T11347.stderr b/testsuite/tests/typecheck/should_fail/T11347.stderr
new file mode 100644
index 0000000..1de61cd
--- /dev/null
+++ b/testsuite/tests/typecheck/should_fail/T11347.stderr
@@ -0,0 +1,2 @@
+dummy stderr:
+here should be something about roles _not_ "No skolem info"
diff --git a/testsuite/tests/typecheck/should_fail/all.T b/testsuite/tests/typecheck/should_fail/all.T
index 4279950..93dd0c7 100644
--- a/testsuite/tests/typecheck/should_fail/all.T
+++ b/testsuite/tests/typecheck/should_fail/all.T
@@ -398,3 +398,4 @@ test('T11112', normal, compile_fail, [''])
 test('ClassOperator', normal, compile_fail, [''])
 test('T11274', normal, compile_fail, [''])
 test('T10619', normal, compile_fail, [''])
+test('T11347', expect_broken(11347), compile_fail, [''])