#14561: Panic on levity polymorphic very unsafe coerce -------------------------------------+------------------------------------- Reporter: goldfire | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.3 Resolution: | Keywords: | LevityPolymorphism Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Ah, hum. Lint rejects the program thus {{{ *** Core Lint errors : in result of Desugar (after optimization) *** <no location info>: warning: [in body of lambda with binder v_B1 :: a_aVG] Levity-polymorphic binder: v_B1 :: (a_aVG :: TYPE r_aVF) *** Offending Program *** levityPolymorphicId :: forall (a :: TYPE r). a -> a [LclIdX] levityPolymorphicId = \ (@ (r_aVF :: RuntimeRep)) (@ (a_aVG :: TYPE r_aVF)) (v_B1 :: a_aVG) -> v_B1 }}} This happens because we want to allow {{{ unsafeCoerce# (x::Int#) }}} and expand it to {{{ x |> (UnsafeCo ...) }}} So we give `unsafeCoerce#` the extremely dodgy unfolding {{{ /\r1 r2 (a:Type r1) (b:Type r2) (x:a). x |> UnsafeCo ... }}} It's dodgy because it has a bad lambda: we can't lambda-bind a levity- polymorphic variable `x`. It only works when `unsafeCoerce#` is saturated. This is tiresome. The only paths forward I can see are * Do not allow `unsafeCoerce` on unboxed values. I don't know how much it is used. * Check that all uses are saturated. Perhaps in the desugarer. The latter would be least destabilising. Any thoughts? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14561#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler