[GHC] #16376: GHCi HEAD silently accepts ill-kinded visible type applications with :type

[GHC]

#16376: GHCi HEAD silently accepts ill-kinded visible type applications with :type
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  highest        |         Milestone:
          Component:  GHCi           |           Version:  8.9
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC accepts
  Unknown/Multiple                   |  invalid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 If you try something nonsensical like `id @Maybe` in GHCi 8.6.3, it will
 be properly rejected:

 {{{
 $ ghci -XTypeApplications
 GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 λ> :t id @Maybe

 <interactive>:1:5: error:
     • Expecting one more argument to ‘Maybe’
       Expected a type, but ‘Maybe’ has kind ‘* -> *’
     • In the type ‘Maybe’
       In the expression: id @Maybe
 }}}

 On GHC HEAD, however, this is seemingly accepted!

 {{{
 $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive -XTypeApplications
 GHCi, version 8.9.20190224: https://www.haskell.org/ghc/  :? for help
 Loaded GHCi configuration from /home/rgscott/.ghci
 λ> :t id @Maybe
 λ>
 }}}

 No error message, just... a newline. Quite bizarre. Even more bizarrely,
 this phenomenon doesn't appear to happen with `:kind`:

 {{{
 λ> :set -XDataKinds -XPolyKinds
 λ> type Id (a :: k) = a
 λ> :k Id @Maybe

 <interactive>:1:5: error:
     • Expecting one more argument to ‘Maybe’
       Expected a type, but ‘Maybe’ has kind ‘* -> *’
     • In the first argument of ‘Id’, namely ‘Maybe’
       In the type ‘Id @Maybe’
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16376>
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