I first noticed something was a bit odd with Constraint when chasing #11334 (closed). Consider this testcase (-O0 is sufficient),

import Data.Typeable

main = do
    print $ typeRep (Proxy :: Proxy Eq)
    print $ typeOf (Proxy :: Proxy Eq)

With ghc-8.0 this will produce,

Eq
Proxy (* -> *) Eq

Notice the second line; GHC seems to be claiming that Eq :: * -> *, which is clearly nonsense.

I believe this issue may be the cause of some of the testsuite failures that I'm seeing on #11011 (closed). Unfortunately I haven't the foggiest where this issue might originate.

See also

• #11621
• #12933 (closed)
• #9547
• #13931 (closed)
• #15918 (about mkCastTy)