#13748: Variables pretty-printed from -ddump-deriv are not scoped properly -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- This bug is present on GHC 8.0.1, 8.0.2, 8.2.1 and HEAD, and originally noted in https://ghc.haskell.org/trac/ghc/ticket/13738#comment:2. Take this code: {{{#!hs {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE PolyKinds #-} {-# OPTIONS_GHC -ddump-deriv #-} module Works where newtype Wrap f a = Wrap (f a) deriving C class C f where c :: f a }}} When you compile it with GHC 8.0.2, you'll get this unsavory output: {{{ GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Works ( Works.hs, interpreted ) ==================== Derived instances ==================== Derived instances: instance forall k_a14U (f_a14V :: k_a14U -> *). Works.C f_a14V => Works.C (Works.Wrap f_a14V) where Works.c = GHC.Prim.coerce @(forall (a_a13F :: k_a14u). f_a13G a_a13F) @(forall (a_a13F :: k_a14u). Works.Wrap f_a13G a_a13F) Works.c GHC.Generics representation types: }}} This is wrong, since the quantified variables in the instance head (`k_a14U` and `f_a14V`) do not match the occurrences that they bind (`k_a14u` and `f_a13G`). This is somewhat easier to see on GHC 8.2.1 or HEAD, since the binding sites are printed without uniques: {{{ GHCi, version 8.3.20170516: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Works ( Works.hs, interpreted ) ==================== Derived instances ==================== Derived class instances: instance forall k (f :: k -> *). Works.C f => Works.C (Works.Wrap f) where Works.c = GHC.Prim.coerce @(forall (a_a1tD :: k_a1uE). f_a1tE a_a1tD) @(forall (a_a1tD :: k_a1uE). Works.Wrap f_a1tE a_a1tD) Works.c Derived type family instances: }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13748 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler