#15289: isUnliftedType GHC panic on pattern with True :: Maybe -------------------------------------+------------------------------------- Reporter: Remi | Owner: (none) Type: bug | Status: new Priority: low | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.4.3 checker) | Keywords: Resolution: | PatternSynonyms 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: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => PatternSynonyms Comment: This also panics on GHC HEAD: {{{ $ /opt/ghc/head/bin/ghc Bug2.hs [1 of 1] Compiling Oops ( Bug2.hs, Bug2.o ) ghc: panic! (the 'impossible' happened) (GHC version 8.5.20180616 for x86_64-unknown-linux): isUnliftedType (Maybe |> {co_aWy}) :: TYPE t_aWE[tau:0] Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler/utils/Outputable.hs:1164:37 in ghc:Outputable pprPanic, called at compiler/types/Type.hs:1934:10 in ghc:Type }}} This appears to be a regression from GHC 8.2.2, in which the same program simply gives an error message: {{{ $ /opt/ghc/8.2.2/bin/ghc Bug2.hs [1 of 1] Compiling Oops ( Bug2.hs, Bug2.o ) Bug2.hs:5:16: error: • Couldn't match expected type ‘Maybe’ with actual type ‘Bool’ • In the pattern: True In the pattern: True :: Maybe In the declaration for pattern synonym ‘What’ | 5 | pattern What = True :: Maybe | ^^^^ Bug2.hs:5:16: error: • Couldn't match expected type ‘Maybe’ with actual type ‘Bool’ • In the expression: True In an equation for ‘What’: What = True | 5 | pattern What = True :: Maybe | ^^^^^^^^^^^^^ Bug2.hs:5:24: error: • Expecting one more argument to ‘Maybe’ Expected a type, but ‘Maybe’ has kind ‘* -> *’ • In the type ‘Maybe’ In a pattern type signature: Maybe In the pattern: True :: Maybe | 5 | pattern What = True :: Maybe | ^^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15289#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler