[GHC] #15144: Type inference regression between GHC 8.0.2 and 8.2.2
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 (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: -------------------------------------+------------------------------------- I observed this when debugging a test case from the `HList` library that works in GHC 8.0.2, but not in GHC 8.2.2 or later. Consider the following minimized example: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE TypeFamilies #-} module Bug where import Data.Coerce import Data.Proxy type family TagR a class TypeIndexed r tr | r -> tr, tr -> r where typeIndexed :: (Coercible (TagR a) s, Functor f) => Proxy a -> (tr (TagR a) -> f (tr (TagR a))) -> r s -> f (r s) typeIndexed' pa x = typeIndexed pa x }}} In GHC 8.0.2, the type of `typeIndexed'` is correctly inferred as: {{{ $ /opt/ghc/8.0.2/bin/ghci Bug.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, modules loaded: Bug. λ> :t typeIndexed' typeIndexed' :: (Coercible s (TagR a), TypeIndexed r tr, Functor f) => Proxy a -> (tr (TagR a) -> f (tr (TagR a))) -> r s -> f (r s) }}} In GHC 8.2.2 and later, however, the inferred type is less general: {{{ $ /opt/ghc/8.4.2/bin/ghci Bug.hs GHCi, version 8.4.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, one module loaded. λ> :t typeIndexed' typeIndexed' :: (TypeIndexed r tr, Functor f) => Proxy a -> (tr (TagR a) -> f (tr (TagR a))) -> r (TagR a) -> f (r (TagR a)) }}} Notice how the `Coercible s (TagR a)` constraint is no longer inferred. Instead, it seems that GHC is inferring the less general constraint `s ~ TagR a`, since `s` has been substituted for `TagR a` in the type `r (TagR a) -> f (r (TagR a))` (whereas in 8.0.2, it was `r s -> f (r s)`). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => TypeFamilies Comment: Here's a slightly smaller test case: {{{#!hs {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} module Bug where import Data.Coerce import Data.Proxy type family F x f :: Coercible (F a) b => Proxy a -> F a -> b f _ = coerce g p x = f p x }}} {{{ $ /opt/ghc/8.0.2/bin/ghci Bug.hs GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, modules loaded: Bug. λ> :t g g :: Coercible b (F a) => Proxy a -> F a -> b }}} {{{ $ /opt/ghc/8.4.2/bin/ghci Bug.hs GHCi, version 8.4.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, one module loaded. λ> :t g g :: Proxy a -> F a -> F a }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:1> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * cc: simonpj (added) Comment: This regression was introduced in commit 1eec1f21268af907f59b5d5c071a9a25de7369c7 (`Another major constraint-solver refactoring`). -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:2> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): What's odd is that things work out alright if I write an explicit type signature: {{{#!hs g :: Coercible (F a) b => Proxy a -> F a -> b g p x = f p x }}} It's only when the type is inferred that it becomes less general. -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:3> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Simon Peyton Jones <simonpj@…>): In [changeset:"ae292c6d1362f34117be75a2553049cec7022244/ghc" ae292c6d/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="ae292c6d1362f34117be75a2553049cec7022244" Do not unify representational equalities This patch is an easy fix to Trac #15144, which was caused by accidentally unifying a representational equality in the unflattener. (The main code in TcInteract was always careful not to do so, but I'd missed the test in the unflattener.) See Note [Do not unify representational equalities] in TcInteract }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:4> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
#15144: Type inference regression between GHC 8.0.2 and 8.2.2 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: normal | Milestone: 8.6.1 Component: Compiler (Type | Version: 8.2.2 checker) | Resolution: fixed | Keywords: TypeFamilies Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: indexed- | types/should_compile/T15144 Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by simonpj): * testcase: => indexed-types/should_compile/T15144 * status: new => closed * resolution: => fixed Comment: Great report, thank you! -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15144#comment:5> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler
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