[GHC] #15601: Unexpected compile error on type level lists with a single element

[GHC]

#15601: Unexpected compile error on type level lists with a single element
-------------------------------------+-------------------------------------
           Reporter:  erikd          |             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:  Incorrect
  Unknown/Multiple                   |  error/warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Tried this with ghc 8.0.2, 8.2.2 and 8.4.3 and all give an almost
 identical error on the following piece of code:
 {{{#!hs

 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE CPP #-}

 {-# OPTIONS_GHC -Wall #-}

 import Data.Singletons.Prelude.List

 #if MIN_VERSION_singletons(2,4,0)
 type (a :++ b) = a ++ b
 #endif

 data MyType = A | B | C | D | E | F

 type TypeList1 = ['A, 'B]

 type TypeList2 = TypeList1 :++ ['C, 'D]

 -- Everything above is fine, but this line:
 type TypeList3 = TypeList2 :++ ['F]
 -- Gives the error:
 --
 --    type-level-list.hs:21:32: error:
 --        • Expected kind ‘[MyType]’, but ‘[ 'F]’ has kind ‘*’
 --        • In the second argument of ‘(:++)’, namely ‘[ 'F]’
 --          In the type ‘TypeList2 :++ [ 'F]’
 --          In the type declaration for ‘TypeList3’
 --       |
 --    22 | type TypeList3 = TypeList2 :++ ['F]

 -- If instead I write it like:
 type TypeList4 = TypeList2 :++ '[F]
 -- I get the warning:
 --
 --    type-level-list.hs:33:34: warning: [-Wunticked-promoted-
 constructors]
 --        Unticked promoted constructor: ‘F’.
 --        Use ‘'F’ instead of ‘F’.

 -- The following actually seems to work, but I don't understand why type
 level
 -- lists containing only one element are different.
 type TypeList5 = TypeList2 :++ '[ 'F]

 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15601>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler