[GHC] #14633: -fwarn-redundant-constraints false positive
#14633: -fwarn-redundant-constraints false positive -------------------------------------+------------------------------------- Reporter: ghorn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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: -------------------------------------+------------------------------------- I had code which compiled cleanly on GHC 8.0.2 with -fwarn-redundant- constraints which now gives a warning on GHC 8.2.2. Here is the code, and my workaround: {{{#!haskell {-# OPTIONS_GHC -Wall -Werror -fwarn-redundant-constraints #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleContexts #-} module Bug ( bug , workaround ) where import GHC.Generics ( D1, Datatype, Meta, Rep, datatypeName ) import Data.Proxy ( Proxy ) -- /home/greghorn/hslibs/ghc82_bug_maybe/Bug.hs:17:1: warning: [-Wredundant-constraints] -- • Redundant constraint: Rep a ~ D1 d p -- • In the type signature for: -- bug :: forall a (d :: Meta) (p :: * -> *). -- (Datatype d, Rep a ~ D1 d p) => -- Proxy a -> String -- | -- 25 | bug :: forall a d p . (Datatype d, Rep a ~ D1 d p) => Proxy a -> String -- | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ bug :: forall a d p . (Datatype d, Rep a ~ D1 d p) => Proxy a -> String bug = const name where name = datatypeName (undefined :: D1 d p b) type family GetD a :: Meta where GetD (D1 d p) = d workaround :: forall a d p . (Datatype (GetD (Rep a)), Rep a ~ D1 d p) => Proxy a -> String workaround = const name where name = datatypeName (undefined :: D1 d p b) }}} I suspect it is a bug because if I remove the "redundant" constraint it no longer typechecks. Here is a minimal setup to reproduce with stack: {{{ name: bug version: 0.0.0.2 license: AllRightsReserved author: Greg Horn maintainer: gregmainland@gmail.com build-type: Simple cabal-version: >=1.10 library exposed-modules: Bug build-depends: base >= 4.7 && < 5 default-language: Haskell2010 }}} {{{ resolver: lts-10.2 compiler-check: newer-minor # Local packages, usually specified by relative directory name packages: - . }}} Alternatively: {{{ git clone https://github.com/ghorn/ghc-redundant-constraint-bug cd ghc-redundant-constraint-bug stack build }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14633 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14633: -fwarn-redundant-constraints false positive -------------------------------------+------------------------------------- Reporter: ghorn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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 simonpj): Here is a much simpler example: {{{ type family F a foo :: forall a b. (Eq a , a ~ F b ) => b -> Bool foo _ = (undefined :: a) == undefined }}} We compiling this we get {{{ T14663.hs:35:1: error: [-Wredundant-constraints, -Werror=redundant- constraints] * Redundant constraint: a ~ F b * In the type signature for: foo :: forall a b. (Eq a, a ~ F b) => b -> Bool | 35 | foo :: forall a b. (Eq a | ^^^^^^^^^^^^^^^^^^^^^^^^... }}} And indeed the equality constraint is redundant: the expression `undefined :: a == undefined` needs `Eq a` but we have that. But if we remove the equality constraint we get this complaint: {{{ T14663.hs:35:8: error: * Could not deduce (Eq a0) from the context: Eq a bound by the type signature for: foo :: forall a b. Eq a => b -> Bool at T14663.hs:(35,8)-(37,33) The type variable `a0' is ambiguous * In the ambiguity check for `foo' <------ NB -------------- To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In the type signature: foo :: forall a b. (Eq a) => b -> Bool | 35 | foo :: forall a b. (Eq a | ^^^^^^^^^^^^^^^^^... }}} Notice that this complaint comes from the '''ambiguity check''' for `foo`. Indeed `foo` really does have an ambiguous type. For example, if have `foo :: Eq a => b -> Bool` and try {{{ foo2 :: Eq c => d -> Bool foo2 = foo }}} we'd fail, because nothing forces the `a` from `foo` to be instantiated to `c` in `foo2`. So the complaint is valid. The solution is to make the type unambiguous, perhaps by adding a proxy parameter: {{{ foo :: forall a b. (Eq a) => Proxy a -> b -> Bool foo _ _ = (undefined :: a) == undefined }}} Now all is well: the type is unambiguous. I suppose you could also try `-XAllowAmbiguousTypes`, but the function really is ambiguous! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14633#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#14633: -fwarn-redundant-constraints false positive -------------------------------------+------------------------------------- Reporter: ghorn | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 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 ghorn): Sounds like GHC 8.2 improved its warnings and exposed a problem with my code -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14633#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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