[GHC] #14042: Misleading error message when type family is used in datatype's return kind

[GHC]

#14042: Misleading error message when type family is used in datatype's return kind
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies, newcomer
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 RyanGlScott):

 Just like type families are rejected in class instances:

 {{{
 > :set -XTypeFamilies
 > type family Foo a
 > instance Show (Foo a)

 <interactive>:4:10: error:
     * Illegal type synonym family application in instance: Foo a
     * In the instance declaration for `Show (Foo a)'
 }}}

 I want the error message for type families in data type return kinds to be
 similarly informative. Something like:

 {{{
 > data Foo :: Id Type
     * Illegal type synonym family application in return kind of data type
 declaration
         Id Type
 }}}

 None of this faffery about non-`*` return kinds, since that's not the real
 issue here.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14042#comment:6>
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