[GHC] #11384: Error says to fix incorrect return type

[GHC]

#11384: Error says to fix incorrect return type
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           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:  GADTs          |  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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 Defining a constructor that returns the wrong type (non-parent type).

 If the constructor is not fully-applied or over-saturated GHC reports an
 error, instead of telling the developer to change the return type:

 {{{
 Prelude> data A
 Prelude> data B a where MkB :: A ()

 <interactive>:2:23: error:
     • Expecting one fewer argument to ‘A’
       Expected kind ‘* -> *’, but ‘A’ has kind ‘*’
     • In the type ‘A ()’
       In the definition of data constructor ‘MkB’
       In the data declaration for ‘B’
 }}}

 Expected:
 1. Suggest GADTs:
 {{{
 • Illegal generalised algebraic data declaration for ‘B’
     (Use GADTs to allow GADTs)
 • In the data declaration for ‘B’
 }}}
 2. Complain about incorrect return type:
 {{{
 • Data constructor ‘MkB’ returns type ‘A’
     instead of an instance of its parent type ‘B a’
 • In the definition of data constructor ‘MkB’
   In the data type declaration for ‘B’
 }}}

 It shouldn't suggest the user fix the incorrect return type

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11384>
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