#14449: Type variables allowed to unify in a partial type signature (PartialTypeSignatures) -------------------------------------+------------------------------------- Reporter: kcsongor | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- During a discussion today, Richard discovered the following bug: The function `f` is accepted in GHC 8.2.1 (and HEAD too): {{{#!hs {-# LANGUAGE PartialTypeSignatures #-} f :: a -> b -> _ f x y = [x, y] }}} with the warning: {{{ Bug.hs:3:16: warning: [-Wpartial-type-signatures] • Found type wildcard ‘_’ standing for ‘[a]’ Where: ‘a’ is a rigid type variable bound by the inferred type of f :: a -> a -> [a] at Bug.hs:4:1-13 • In the type signature: f :: a -> b -> _ | 3 | f :: a -> b -> _ | ^ Ok, 1 module loaded. }}} This is a regression compared to 8.0.1, where the following error is produced (rightly): {{{ Bug.hs:4:12: error: • Couldn't match expected type ‘a’ with actual type ‘b’ ‘b’ is a rigid type variable bound by the inferred type of f :: a -> b -> [a] at Bug.hs:3:6 ‘a’ is a rigid type variable bound by the inferred type of f :: a -> b -> [a] at Bug.hs:3:6 • In the expression: y In the expression: [x, y] In an equation for ‘f’: f x y = [x, y] • Relevant bindings include y :: b (bound at Bug.hs:4:5) x :: a (bound at Bug.hs:4:3) f :: a -> b -> [a] (bound at Bug.hs:4:1) Failed, modules loaded: none. }}} Note that the inferred type of `f` is still correct: `f :: a -> a -> a`, but the type variables `a` and `b` are allowed to unify during inference, and they shouldn't be. If I understood correctly, this implies that when inferring the type of functions with partial type signatures, the type variables are (wrongly) treated as `SigTv`s, instead of skolem constants. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14449 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler