+[Silly Type Synonyms]
+
+Consider this:
+ type C u a = u -- Note 'a' unused
+
+ foo :: (forall a. C u a -> C u a) -> u
+ foo x = ...
+
+ bar :: Num u => u
+ bar = foo (\t -> t + t)
+
+* From the (\t -> t+t) we get type {Num d} => d -> d
+ where d is fresh.
+
+* Now unify with type of foo's arg, and we get:
+ {Num (C d a)} => C d a -> C d a
+ where a is fresh.
+
+* Now abstract over the 'a', but float out the Num (C d a) constraint
+ because it does not 'really' mention a. (see Type.tyVarsOfType)
+ The arg to foo becomes
+ /\a -> \t -> t+t
+
+* So we get a dict binding for Num (C d a), which is zonked to give
+ a = ()
+
+* Then the /\a abstraction has a zonked 'a' in it.
+
+All very silly. I think its harmless to ignore the problem.
+