Fix #11473.

I've added a check in the zonker for representation polymorphism.
I don't like having this be in the zonker, but I don't know where
else to put it. It can't go in TcValidity, because a clever enough
user could convince the solver to do bogus representation polymorphism
even though there's nothing obviously wrong in what they wrote.
Note that TcValidity doesn't run over *expressions*, which is where
this problem arises.

In any case, the check is simple and it works.

test case: dependent/should_fail/T11473

3 files changed, 35 insertions, 0 deletions

diff --git a/testsuite/tests/dependent/should_fail/T11473.hs b/testsuite/tests/dependent/should_fail/T11473.hs
new file mode 100644
index 0000000000..12d95caac6
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T11473.hs
@@ -0,0 +1,27 @@
+{-# LANGUAGE PolyKinds, TypeFamilies, MagicHash, DataKinds, TypeInType, RankNTypes #-}
+
+module T11473 where
+
+import GHC.Exts
+import GHC.Types
+
+type family Boxed (a :: k) :: *
+type instance Boxed Char# = Char
+type instance Boxed Char  = Char
+
+class BoxIt (a :: TYPE lev) where
+    boxed :: a -> Boxed a
+
+instance BoxIt Char# where boxed x = C# x
+instance BoxIt Char  where boxed = id
+
+hello :: forall (r :: RuntimeRep). forall (a :: TYPE r). BoxIt a => a -> Boxed a
+hello x = boxed x
+{-# NOINLINE hello #-}
+
+main :: IO ()
+main = do
+    print $ boxed 'c'#
+    print $ boxed 'c'
+    print $ hello 'c'
+    print $ hello 'c'#
diff --git a/testsuite/tests/dependent/should_fail/T11473.stderr b/testsuite/tests/dependent/should_fail/T11473.stderr
new file mode 100644
index 0000000000..7a7cc32871
--- /dev/null
+++ b/testsuite/tests/dependent/should_fail/T11473.stderr
@@ -0,0 +1,7 @@
+
+T11473.hs:19:7: error:
+    The following variable has an unknown runtime representation:
+        Var name: x
+        Var type: a
+     Type's kind: TYPE r
+    Perhaps add a type or kind signature to fix the representation.
diff --git a/testsuite/tests/dependent/should_fail/all.T b/testsuite/tests/dependent/should_fail/all.T
index a90b7bbcdc..c75a9c6a56 100644
--- a/testsuite/tests/dependent/should_fail/all.T
+++ b/testsuite/tests/dependent/should_fail/all.T
@@ -12,3 +12,4 @@ test('T11407', normal, compile_fail, [''])
 test('T11334', normal, compile_fail, [''])
 test('InferDependency', normal, compile_fail, [''])
 test('KindLevelsB', normal, compile_fail, [''])
+test('T11473', normal, compile_fail, [''])