[GHC] #13838: -Wdeferred-type-errors; ghc: panic!; VoidRep; ((() -> ()) :: *) ~# (IO Any :: *)

[GHC]

#13838: -Wdeferred-type-errors; ghc: panic!; VoidRep; ((() -> ()) :: *) ~# (IO Any
:: *)
-------------------------------------+-------------------------------------
        Reporter:  harry             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Compile-time      |  Unknown/Multiple
  crash or panic                     |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 Thanks for the bug report.

 Interestingly, this program no longer panics with just `-fdefer-type-
 errors` on GHC 8.2.1 or HEAD:

 {{{
 $ /opt/ghc/8.2.1/bin/ghc Bug.hs -fdefer-type-errors
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )

 Bug.hs:2:1: warning: [-Wdeferred-type-errors]
     • Couldn't match expected type ‘IO t0’ with actual type ‘() -> ()’
     • Probable cause: ‘main’ is applied to too few arguments
       In the expression: main
       When checking the type of the IO action ‘main’
   |
 2 | main = () ()
   | ^

 Bug.hs:2:8: warning: [-Wdeferred-type-errors]
     • Couldn't match expected type ‘() -> () -> ()’
                   with actual type ‘()’
     • The function ‘()’ is applied to one argument,
       but its type ‘()’ has none
       In the expression: () ()
       In an equation for ‘main’: main = () ()
   |
 2 | main = () ()
   |        ^^^^^
 Linking Bug ...
 }}}

 (It also appears to be `-dcore-lint`-safe.)

 However, the output isn't exactly what I'd expect from a type-incorrect
 program that was compiled with `-fdefer-type-errors`:

 {{{
 $ ./Bug
 Bug: main thread exited (uncaught exception)
 }}}

 I would have expected something like this:

 {{{
 $ ./Bug
 Bug: Bug.hs:2:1: error:
     • Couldn't match expected type ‘IO t0’ with actual type ‘() -> ()’
     • Probable cause: ‘main’ is applied to too few arguments
       In the expression: main
       When checking the type of the IO action ‘main’

 Bug: Bug.hs:2:8: error:
     • Couldn't match expected type ‘() -> () -> ()’
                   with actual type ‘()’
     • The function ‘()’ is applied to one argument,
       but its type ‘()’ has none
       In the expression: () ()
       In an equation for ‘main’: main = () ()
 }}}

 I'll investigate which commit fixed the panic.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13838#comment:1>
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