#8129: Constraint solver panic due to derived type function equality --------------------------------------------+------------------------------ Reporter: adamgundry | Owner: Type: bug | Status: new Priority: high | Milestone: Component: Compiler (Type checker) | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time crash | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Changes (by adamgundry): * priority: normal => high Comment: The above example, without `-ddump-tc-trace`, now causes the latest HEAD to panic with: {{{ ghc-stage2: panic! (the 'impossible' happened) (GHC version 7.7.20130915 for i386-unknown-linux): ASSERT failed! file compiler/typecheck/TcMType.lhs line 809 [D] _ :: main:Main.F{tc rpc} x_aAu{tv} [tau[0]] ghc-prim:GHC.Types.~{(w) tc 31Q} y_aAv{tv} [tau[0]] (CFunEqCan) }}} It looks like the problem was introduced in e365d4963f1061878269502f256b4a56ca273a78 (the fix to #8262). The new code calls `solveWantedsTcMWithEvBinds` with `solve_wanteds`, rather than `solve_wanteds_and_drop`. But `solveWantedsTcMWithEvBinds` calls `zonkWC`, which works only with wanted (not derived) constraints. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8129#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler