[GHC] #12761: Type aliases for TypeError constrains fire during compilation time

[GHC]

#12761: Type aliases for TypeError constrains fire during compilation time
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           Reporter:  danilo2        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 Hello!

 The following code should compile fine:

 {{{
 module Main where

 import GHC.TypeLits (ErrorMessage(Text, ShowType, (:<>:)), TypeError)

 type MyError a b = TypeError (ShowType a :<>: ShowType b)

 main :: IO ()
 main = return ()

 }}}

 but it raises an error during compilation:

 {{{
 Main.hs:5:1: error:
     • ab
     • In the type synonym declaration for ‘MyError’
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12761>
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