#15052: DeriveAnyClass instances may skip TypeError constraints -------------------------------------+------------------------------------- Reporter: jcpetruzza | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: | CustomTypeErrors 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): Here is the difference between these two programs. In the first one: {{{#!hs data TS = TSL | TSR deriving (Gen.Generic, C) }}} You are requesting that GHC come up with the `???` in the following instance: {{{#!hs instance ??? => C TS }}} Because `C` has a superclass constraint, `TypeError (...)`, it infers that for `???`. If you had picked different superclass constraints for `C`, such as `class (Eq a, Show a) => C a`, then GHC would have inferred `(Eq a, Show a)` for `???`. In the second example, you are putting the task of coming up with the context in your own hands. You have boldly declared that the instance context should be `()`! As a result, GHC tries to unify `TypeError (...)` with `()`, which causes the type error to trigger. Boom. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15052#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler