Constraint deduction failure

I get a strange error message like this

[1 of 1] Compiling Main             ( 08-10.hs, interpreted )

08-10.hs:254:33:
    Could not deduce (KnownLoc (ent par1 a))
      arising from a use of ‘Hidden'’
    from the context (List ConfFamily ts)
      bound by the type signature for
                 locAware :: List ConfFamily ts =>
                             (forall (i :: k).
                              KnownLoc (ent par i) =>
                              ConfFamily (Facility (ent par i)) ts)
                             -> Con ConfFamily (On ent par)
      at 08-10.hs:251:13-160
    or from (ent par a ~ 'Monitor par1 mp, KnownLoc ('Monitor par1 mp))
      bound by a pattern with constructor
                 MonitorF :: forall (par :: Entity) (mp :: MonitorPoint).
                             KnownLoc ('Monitor par mp) =>
                             String -> String -> Observer -> Bool -> Facility ('Monitor par mp),
               in an equation for ‘h’
      at 08-10.hs:254:19-28
    In the expression: Hidden' (d f)
    In an equation for ‘h’: h (Hide f@(MonitorF {})) = Hidden' (d f)
    In an equation for ‘locAware’:
        locAware descr
          = Abstr h
          where
              h :: (ent `On` par) -> ConfFamily (ent `On` par) ts
              h (Hide f@(MonitorF {})) = Hidden' (d f)
              d ::
                KnownLoc (ent par i) =>
                Facility (ent par i) -> ConfFamily (Facility (ent par i)) ts
              d _ = descr
Failed, modules loaded: none.

I have many language extensions active, such as, DataKinds, ScopedTypeVariables, etc.

If somebody says this indeed looks like a bug, then I'll try to post a minimal reproduction case.

• * My reasoning goes like this **

Here are the relevant facts that GHC figured out from a GADT pattern match: (ent par a ~ 'Monitor par1 mp, KnownLoc ('Monitor par1 mp))

From this I discover these equalities:

ent === 'Monitor    (*)
par === par1        (**)
a === mp            (***)

Substituring (*)^-1^ and (***)^-1^ into the given KnownLoc ('Monitor par1 mp) results in the desired KnownLoc (ent par1 a)

Looks like a bug, right?

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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