[GHC] #15941: Pretty-printing of invisible arguments to (->)

[GHC]

#15941: Pretty-printing of invisible arguments to (->)
-------------------------------------+-------------------------------------
        Reporter:  monoidal          |                Owner:  RyanGlScott
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Compiler          |              Version:  8.6.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by RyanGlScott):

 * version:  8.7 => 8.6.2


Comment:

 I set the version of this ticket to 8.7, mistakenly thinking that this was
 a regression introduced in commit
 f5d2083807a03c57f194fcc3a7baf82e34aad524. But as it turns out, this bug
 has been lurking even before that! In GHC 8.6.2, you get this:

 {{{
 > :i T
 type T =
   'GHC.Types.LiftedRep -> 'GHC.Types.LiftedRep :: * -> * -> *
         -- Defined at <interactive>:3:1
 }}}

 Another observation: this is not unique to `(->)` (which is what I
 originally thought, since there are many special cases for function arrows
 throughout the codebase). Any infix type constructor with two invisible
 arguments will do, as the following GHCi session shows:

 {{{
 > import GHC.Generics
 > type T2 = (:.:)
 > :i T2
 type T2 = * :.: * :: (* -> *) -> (* -> *) -> * -> *
         -- Defined at <interactive>:11:1
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15941#comment:2>
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