Type equality makes type variable untouchable

This doesn't look right:

{-# LANGUAGE RankNTypes, TypeFamilies #-}

type family TF (m :: * -> *)

run1 :: (forall m . Monad m => m a) -> a
run1 = undefined

run2 :: (forall m . (Monad m, TF m ~ ()) => m a) -> a
run2 = undefined

-- this works
x1 = run1 (return ())

-- this fails
x2 = run2 (return ())
{-
    Couldn't match expected type ‘a’ with actual type ‘()’
      ‘a’ is untouchable
        inside the constraints (Monad m, TF m ~ ())
        bound by a type expected by the context:
                   (Monad m, TF m ~ ()) => m a
        at untouchable.hs:15:6-21
      ‘a’ is a rigid type variable bound by
          the inferred type of x2 :: a at untouchable.hs:15:1
    Relevant bindings include x2 :: a (bound at untouchable.hs:15:1)
    In the first argument of ‘return’, namely ‘()’
    In the first argument of ‘run2’, namely ‘(return ())’
-}

I would expect x2 to type check just like x1 does.

Trac metadata

Trac field	Value
Version	7.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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