[GHC] #12928: Too easy to trigger CUSK condition using TH

[GHC]

#12928: Too easy to trigger CUSK condition using TH
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        Reporter:  int-index         |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by int-index):

 How come we don't have a notion of "complete user-supplied type" to
 support polymorphic recursion at term level?

 Also I've noticed that with GHC HEAD the error message is way worse:

 {{{
 Cusk.hs:27:15: error:
     • Illegal type synonym family application in instance:
         Proxy a1627472612
     • In the type instance declaration for ‘Apply’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12928#comment:2>
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