[GHC] #14778: FunDep origin not correctly attributed

[GHC]

#14778: FunDep origin not correctly attributed
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           Reporter:  simonpj        |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.2
           Keywords:  FunDeps        |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In `TcInteract.improveTopFunEqs` we do not give the derived constraints an
 origin of `FunDepOrigin`.  As a result, `dropDerivedSimples` will discard
 even an insoluble constraint arising from a fundep.  In contrast class
 instance fundeps do have `FunDepOrigin` and are kept.

 This is at least inconsistent.  And we get less good error messages.

 Shows up in tests `T13651, `TR8450`, `T13506`, `T14325`.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14778>
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