[GHC] #15899: Testcase tcfail158 is broken

[GHC]

#15899: Testcase tcfail158 is broken
-------------------------------------+-------------------------------------
        Reporter:  RolandSenn        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.8.1
       Component:  Test Suite        |              Version:  8.7
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12906            |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Here is a somewhat simplified version of the issue:

 {{{#!hs
 module Bug where

 f, g :: Maybe
 f = Nothing
 g = undefined
 }}}

 In GHC 8.6.2, this simply errors with:

 {{{
 $ ~/Software/haskell/ghc-8.6.2/bin/ghc Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:3:9: error:
     • Expecting one more argument to ‘Maybe’
       Expected a type, but ‘Maybe’ has kind ‘* -> *’
     • In the type signature: f :: Maybe
   |
 3 | f, g :: Maybe
   |         ^^^^^
 }}}

 But in GHC HEAD (as of commit df570d920fa66db631f936fa377e598fe92bd2a1),
 we get many more errors:

 {{{
 $ ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:3:9: error:
     • Expecting one more argument to ‘Maybe’
       Expected a type, but ‘Maybe’ has kind ‘* -> *’
     • In the type signature: f :: Maybe
   |
 3 | f, g :: Maybe
   |         ^^^^^

 Bug.hs:4:5: error:
     • Couldn't match expected type ‘Maybe’ with actual type ‘Maybe a1’
     • In the expression: Nothing
       In an equation for ‘f’: f = Nothing
   |
 4 | f = Nothing
   |     ^^^^^^^

 Bug.hs:5:5: error:
     • Couldn't match kind ‘* -> *’ with ‘*’
       When matching types
         a0 :: *
         Maybe :: * -> *
     • In the expression: undefined
       In an equation for ‘g’: g = undefined
   |
 5 | g = undefined
   |     ^^^^^^^^^
 }}}

 I'll try tracking down which commit introduced this regression when I get
 a chance.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15899#comment:3>
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