[GHC] #15899: Testcase tcfail158 is broken

15 Nov 2018

      #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
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Other
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Test testsuite/tests/typecheck/should_fail/tcfail158.hs contains the
 following code:

 {{{#!hs
 {-# LANGUAGE ExplicitForAll #-}

 -- This one actually crashed in 6.4.1
 -- There's a kind error in the signature for bar,
 -- and we were recovering, and then crashing when we found
 -- a scoped type variable not in scope

  data Val v sm = Val
  foo :: forall v sm. Val v sm
  foo = undefined
    where foo1 :: Val v sm
          foo1 = bar
  -- Correct type signature: bar :: forall v sm. Val v sm
  bar :: forall v. Val v
  bar = undefined foo
 }}}

 The expected test result in
 testsuite/tests/typecheck/should_fail/tcfail158.stderr is:

 {{{
 tcfail158.hs:1:1: error:
     The IO action ‘main’ is not defined in module ‘Main’
 }}}

 == Issues:

 1. With a very high probability, the intention of this test was not to
 test on the missing main function. The intention was to test the error
 message for the type signature of function ''bar'' .

 2. The code for the definitions and functions start in column 2 and not in
 column 1.

 3. With an added main function GHC Versions 8.0, 8.2, 8.4 and 8.6 report
 the following error:

 {{{
 tcfail158.hs:17:18: error:
     • Expecting one more argument to ‘Val v’
       Expected a type, but ‘Val v’ has kind ‘* -> *’
     • In the type signature:
         bar :: forall v. Val v
 }}}

 4. GHC HEAD version 8.7.20181112, however, reports:

 {{{
 tcfail158.hs:15:17: error:
     • Couldn't match expected type ‘Val v1 sm1’
                   with actual type ‘Val v0’
     • In the expression: bar
       In an equation for ‘foo1’: foo1 = bar
       In an equation for ‘foo’:
           foo
             = undefined
             where
                 foo1 :: Val v sm
                 foo1 = bar
     • Relevant bindings include
         foo1 :: Val v1 sm1 (bound at tcfail158.hs:15:10)

 tcfail158.hs:17:18: error:
     • Expecting one more argument to ‘Val v’
       Expected a type, but ‘Val v’ has kind ‘* -> *’
     • In the type signature: bar :: forall v. Val v
                ^^^^^
 tcfail158.hs:18:7: error:
     • Couldn't match kind ‘* -> *’ with ‘*’
       When matching types
         t0 :: *
         Val v :: * -> *
     • In the expression: undefined foo
       In an equation for ‘bar’: bar = undefined foo
     • Relevant bindings include
         bar :: Val v (bound at tcfail158.hs:18:1)

 }}}

 GHC HEAD reports errors in 3 different lines 15, 17 and 18. Fix the type
 signature in line 17 and GHC compiles successfully. The error messages
 produced by GHC HEAD are very confusing!

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15899
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