#15325: Panic in getIdFromTrivialExpr with -fdefer-type-errors -------------------------------------+------------------------------------- Reporter: dramforever | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 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: -------------------------------------+------------------------------------- == Steps to reproduce: Put this in `bug.hs`: (It's a very failed attempt to make a polymorphic list maker function.) {{{#!hs {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} module PolyvariadicFunctions where class PolyList e a where polyList :: ([e] -> [e]) -> a -- instance PolyList e [e] where -- polyList dl = dl [] instance (e ~ e2, PolyList e a) => PolyList e (e2 -> a) where polyList dl x = polyList ((x :) . dl) plh :: [Integer] plh = polyList 1 2 3 4 5 }}} Load it in GHCi with `-fdefer-type-errors` to get the following output. Note the 'panic' in the end, and also note that the last `>` line is a no- module-loaded GHCi prompt. The question marks `?` seem to be encoding related and not relevant. This panic not seem to occur with GHC the compiler. The rest of the error messages seem otherwise identical. {{{ GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :set -fdefer-type-errors Prelude> :l bug.hs [1 of 1] Compiling PolyvariadicFunctions ( bug.hs, interpreted ) bug.hs:17:7: warning: [-Wdeferred-type-errors] ? No instance for (PolyList t0 [Integer]) arising from a use of ‘polyList’ ? In the expression: polyList 1 2 3 4 5 In an equation for ‘plh’: plh = polyList 1 2 3 4 5 | 17 | plh = polyList 1 2 3 4 5 | ^^^^^^^^^^^^^^^^^^ bug.hs:17:16: warning: [-Wdeferred-type-errors] ? No instance for (Num ([t0] -> [t0])) arising from the literal ‘1’ (maybe you haven't applied a function to enough arguments?) ? In the first argument of ‘polyList’, namely ‘1’ In the expression: polyList 1 2 3 4 5 In an equation for ‘plh’: plh = polyList 1 2 3 4 5 | 17 | plh = polyList 1 2 3 4 5 | ^ bug.hs:17:18: warning: [-Wdeferred-type-errors] ? Ambiguous type variable ‘t0’ arising from the literal ‘2’ prevents the constraint ‘(Num t0)’ from being solved. Probable fix: use a type annotation to specify what ‘t0’ should be. These potential instances exist: instance Num Integer -- Defined in ‘GHC.Num’ instance Num Double -- Defined in ‘GHC.Float’ instance Num Float -- Defined in ‘GHC.Float’ ...plus two others (use -fprint-potential-instances to see them all) ? In the second argument of ‘polyList’, namely ‘2’ In the expression: polyList 1 2 3 4 5 In an equation for ‘plh’: plh = polyList 1 2 3 4 5 | 17 | plh = polyList 1 2 3 4 5 | ^ ghc.exe: panic! (the 'impossible' happened) (GHC version 8.4.3 for x86_64-unknown-mingw32): getIdFromTrivialExpr case $dNum_a1Be of wild_00 { } Call stack: CallStack (from HasCallStack): callStackDoc, called at compiler\utils\Outputable.hs:1150:37 in ghc:Outputable pprPanic, called at compiler\\coreSyn\\CoreUtils.hs:883:18 in ghc:CoreUtils Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug

}}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15325 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler