[GHC] #15142: GHC HEAD regression: tcTyVarDetails

[GHC]

#15142: GHC HEAD regression: tcTyVarDetails
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies, CUSKs
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > For non-CUSKs, we can safely simply make T :: kappa, without looking at
 the declaration at all!

 Actually this is not true, as things stand.  This accepted:
 {{{
 data T k (a::k) = MkT (Proxy (T * Int)) (Proxy (T (*->*) Maybe))
 }}}
 But `T` does not have a CUSK, and if we assigned it the mono-kind `kappa`,
 the recursive definition would be rejected.

 So currently we are cleverly (and somewhat accidentally) accepting a
 recursive definition with a partial kind signature.

 I think we should just reject this definition unless you write a CUSK.
 Specifying exactly what is and what is not accepted would be ...
 difficult.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15142#comment:10>
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