#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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler