#12797: Default Rules stop working when providing some constraints -------------------------------------+------------------------------------- Reporter: danilo2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | 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: | -------------------------------------+------------------------------------- Comment (by simonpj): I took a quick look. It's because `TcSimplify.findDefaultableGroups` uses `TcSimplify.approximateWC`; and the latter is also used when inferring the most general type of a function that lacks a type signature. And `approximateWC` has the following note: {{{ Note [ApproximateWC] ~~~~~~~~~~~~~~~~~~~~ 1. We do *not* float anything out if the implication binds equality constraints, because that defeats the OutsideIn story. Consider data T a where TInt :: T Int MkT :: T a f TInt = 3::Int We get the implication (a ~ Int => res ~ Int), where so far we've decided f :: T a -> res We don't want to float (res~Int) out because then we'll infer f :: T a -> Int which is only on of the possible types. (GHC 7.6 accidentally *did* float out of such implications, which meant it would happily infer non-principal types.) }}} But in the case of ''defaulting'' we ''do'' want to infer a less-than- most-general type; and that's just what is happening here. The fix is easy: give a boolean flag to `approximateWC`. I'll do that soon. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12797#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler