#15800: Overlapping instances error with single instance -------------------------------------+------------------------------------- Reporter: roland | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{ {-# LANGUAGE MultiParamTypeClasses, FlexibleContexts, FlexibleInstances #-} module Bug where class C a b instance C a Int x :: () x = undefined :: C a Int => () }}} {{{ ghc -c Bug.hs Bug.hs:10:18: error: • Overlapping instances for C a0 Int Matching givens (or their superclasses): C a Int bound by an expression type signature: forall a. C a Int => () at Bug.hs:10:18-30 Matching instances: instance C a Int -- Defined at Bug.hs:7:10 (The choice depends on the instantiation of ‘a0’) • In the ambiguity check for an expression type signature To defer the ambiguity check to use sites, enable AllowAmbiguousTypes In an expression type signature: C a Int => () In the expression: undefined :: C a Int => () | 10 | x = undefined :: C a Int => () | ^^^^^^^^^^^^^ }}} The "matching instances" bit of the error messages only lists a single instance. Doesn't it take at least two instances for something to overlap? Also, following the algorithm laid out in the user guide (section "Overlapping Instances"), it appears this program should be accepted. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15800 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler