#10625: Spurious unused quantified type variable warning with ExistentialQuantification -------------------------------------+------------------------------------- Reporter: | Owner: RyanGlScott | Status: new Type: bug | Milestone: Priority: normal | Version: 7.10.1 Component: Compiler | Operating System: Unknown/Multiple Keywords: | Type of failure: Incorrect Architecture: | warning at compile-time Unknown/Multiple | Blocked By: Test Case: | Related Tickets: #5331 Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- When using an existentially quantified type variable that shadows another variable in a data declaration, {{{-Wall}}} produces more warnings than necessary: {{{ $ ghci -XExistentialQuantification -Wall GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help λ> data Ex a = forall a. Ex a <interactive>:2:20: Warning: Unused quantified type variable ‘a’ In the definition of data constructor ‘Ex’ <interactive>:2:20: Warning: This binding for ‘a’ shadows the existing binding bound at <interactive>:2:9 }}} The unused quantified type variable warning seems wrong, since it ''is'' being used (as the shadowing warning indicates). Curiously, this warning doesn't seem to appear when doing something similar with {{{RankNTypes}}}: {{{ $ ghci -XRankNTypes -Wall GHCi, version 7.10.1: http://www.haskell.org/ghc/ :? for help λ> data Ex a = Ex (forall a. a) <interactive>:2:24: Warning: This binding for ‘a’ shadows the existing binding bound at <interactive>:2:9 }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10625 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler