[GHC] #15367: Misleading error message when unboxed tuples used in sums without -XUnboxedTuples

[GHC]

#15367: Misleading error message when unboxed tuples used in sums without
-XUnboxedTuples
-------------------------------------+-------------------------------------
           Reporter:  osa1           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 $ cat lib.hs
 {-# LANGUAGE UnboxedSums #-}

 module Lib where

 type Foo = (# (# Int, Int #) | Int #)

 $ ghc lib.hs
 [1 of 1] Compiling Lib              ( lib.hs, lib.o )

 lib.hs:5:1: error:
     • Illegal unboxed tuple type as function argument: (# Int, Int #)
     • In the type synonym declaration for ‘Foo’
   |
 5 | type Foo = (# (# Int, Int #) | Int #)
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 But there's nothing illegal about this type, we just need to enable
 `UnboxedTuples`. The error message is misleading.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15367>
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