#15431: Coercible and Existential types don't play nicely -------------------------------------+------------------------------------- Reporter: NioBium | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Roles Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #14333 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => Roles * related: => #14333 Comment: Hm, this is interesting. This can be minimized to the following examples, which are slight variations of each other: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE GADTs #-} module Bug where import Data.Coerce import Data.Functor.Identity g1 :: Coercible (t a) Int => t a -> Int g1 = coerce g2 :: Coercible Int (t a) => t a -> Int g2 = coerce }}} `g1` typechecks, but `g2` doesn't! {{{ $ /opt/ghc/8.4.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:12:6: error: • Couldn't match representation of type ‘t a’ with that of ‘Int’ arising from a use of ‘coerce’ • In the expression: coerce In an equation for ‘g2’: g2 = coerce • Relevant bindings include g2 :: t a -> Int (bound at Bug.hs:12:1) | 12 | g2 = coerce | ^^^^^^ }}} I'm not sure if this is related to #14333 or not. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15431#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler