-Wdeferred-type-errors on a program with main not of type IO () yields "main thread exited (uncaught exception)"

My Compiler told me to report this so here you go:

[1 of 1] Compiling Main             ( ghc_bug.hs, interpreted )

ghc_bug.hs:3:1: 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'

ghc_bug.hs:3:8: 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 = () ()
ghc: panic! (the 'impossible' happened)
  (GHC version 8.0.1 for x86_64-unknown-linux):
        corePrepPgm
  [False] cobox_r1a0 = typeError
                         @ 'VoidRep
                         @ ((() -> ()) :: *) ~# (IO Any :: *)
                         "ghc_bug.hs:3:1: error:\n\
                         \    * Couldn't match expected type `IO t0' with actual type `() -> ()'\n\
                         \    * Probable cause: `main' is applied to too few arguments\n\
                         \      In the expression: main\n\
                         \      When checking the type of the IO action `main'\n\
                         \(deferred type error)"#

Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug

main :: () -> ()
main = () ()

which that in "ghc_bug.hs" "ghc ghc_bug.hs -fdefer-type-errors -dcore-lint -v" gives me the following:

Glasgow Haskell Compiler, Version 8.0.1, stage 2 booted by GHC version 7.10.3
Using binary package database: /usr/lib/ghc-8.0.1/package.conf.d/package.cache
Using binary package database: /home/harry/.ghc/x86_64-linux-8.0.1/package.conf.d/package.cache
loading package database /usr/lib/ghc-8.0.1/package.conf.d
loading package database /home/harry/.ghc/x86_64-linux-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
Hsc static flags: 
loading package database /usr/lib/ghc-8.0.1/package.conf.d
loading package database /home/harry/.ghc/x86_64-linux-8.0.1/package.conf.d
wired-in package ghc-prim mapped to ghc-prim-0.5.0.0
wired-in package integer-gmp mapped to integer-gmp-1.0.0.1
wired-in package base mapped to base-4.9.0.0
wired-in package rts mapped to rts-1.0
wired-in package template-haskell mapped to template-haskell-2.11.0.0
wired-in package ghc mapped to ghc-8.0.1
wired-in package dph-seq not found.
wired-in package dph-par not found.
*** Chasing dependencies:
Chasing modules from: *ghc_bug.hs
!!! Chasing dependencies: finished in 0.42 milliseconds, allocated 0.211 megabytes
Stable obj: []
Stable BCO: []
Ready for upsweep
  [NONREC
      ModSummary {
        ms_hs_date = ****-**-** **:**:**.********** ***
        ms_mod = Main,
        ms_textual_imps = [(Nothing, Prelude)]
        ms_srcimps = []
      }]
*** Deleting temp files:
Deleting: 
compile: input file ghc_bug.hs
*** Checking old interface for Main:
[1 of 1] Compiling Main             ( ghc_bug.hs, ghc_bug.o )
*** Parser [Main]:
!!! Parser [Main]: finished in 0.14 milliseconds, allocated 0.089 megabytes
*** Renamer/typechecker [Main]:
!!! Renamer/typechecker [Main]: finished in 28.70 milliseconds, allocated 14.769 megabytes
*** Desugar [Main]:
Result size of Desugar (after optimization)
  = {terms: 19, types: 52, coercions: 4}
*** Core Linted result of Desugar (after optimization):
*** Core Lint errors : in result of Desugar (after optimization) ***
<no location info>: warning:
    [RHS of cobox_aOD :: ((() -> ()) :: *) ~# (IO Any :: *)]
    The type of this binder is primitive: cobox_aOD
    Binder's type: ((() -> ()) :: *) ~# (IO Any :: *)
*** Offending Program ***
main :: () -> ()
[LclIdX, Str=DmdType]
main =
  case typeError
        @ 'VoidRep
        @ (() :: *) ~# ((() -> () -> ()) :: *)
        "ghc_bug.hs:2:8: error:\n\
        \    * Couldn't match expected type `() -> () -> ()'\n\
        \                  with actual type `()'\n\
        \    * The function `()' is applied to one argument,\n\
        \      but its type `()' has none\n\
        \      In the expression: () ()\n\
        \      In an equation for `main': main = () ()\n\
        \(deferred type error)"#
  of cobox_aOE { __DEFAULT ->
  (()
  `cast` (Sub cobox_aOE :: (() :: *) ~R# ((() -> () -> ()) :: *)))
    ()
  }

$trModule :: Module
[LclIdX, Str=DmdType]
$trModule = Module (TrNameS "main"#) (TrNameS "Main"#)

cobox_aOD :: ((() -> ()) :: *) ~# (IO Any :: *)
[LclId[CoVarId], Str=DmdType]
cobox_aOD =
  typeError
    @ 'VoidRep
    @ ((() -> ()) :: *) ~# (IO Any :: *)
    "ghc_bug.hs:2:1: error:\n\
    \    * Couldn't match expected type `IO t0' with actual type `() -> ()'\n\
    \    * Probable cause: `main' is applied to too few arguments\n\
    \      In the expression: main\n\
    \      When checking the type of the IO action `main'\n\
    \(deferred type error)"#

main :: IO Any
[LclIdX, Str=DmdType]
main =
  runMainIO
    @ Any
    (main
    `cast` (Sub cobox_aOD :: ((() -> ()) :: *) ~R# (IO Any :: *)))

*** End of Offense ***


<no location info>: error: 
Compilation had errors


*** Deleting temp files:
Deleting: 
*** Deleting temp dirs:
Deleting: 
 

and gcc version is: gcc version 7.1.1 20170516 (GCC)

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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