
#14237: -Wredundant-constraints incorrectly warns about required type equality constraints in default signatures -------------------------------------+------------------------------------- Reporter: lexi.lambda | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by lexi.lambda): This actually seems worse than I thought—GHC warns even when type equalities are clearly required. It seems like perhaps `-Wredundant- constraints` in GHC 8.2.1 treats all type equalities as redundant? For example, this program produces a warning: {{{#!hs import GHC.Exts (IsList(..)) nums :: (Num a, IsList t, Item t ~ a) => t nums = fromList [1, 2, 3] }}} {{{ /private/tmp/redundant-default- constraint/src/RedundantDefaultConstraints.hs:9:1: warning: [-Wredundant- constraints] • Redundant constraint: Item t ~ a • In the type signature for: nums :: forall a t. (Num a, IsList t, Item t ~ a) => t | 9 | nums :: (Num a, IsList t, Item t ~ a) => t | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14237#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler