#15472: GHC HEAD type inference regression post-"Remove decideKindGeneralisationPlan" -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: closed Priority: highest | Milestone: 8.8.1 Component: Compiler (Type | Version: 8.5 checker) | Resolution: invalid | Keywords: TypeInType 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 RyanGlScott): To see why the program in comment:7 is going awry, suppose the `sGo` function were defined at the top level: {{{#!hs sGo :: x -> Proxy LetGo sGo _ = foo }}} This will not kind-check, before or after the offending commit mentioned above. That's because we always kind-generalize for top-level definitions, so the return kind of `LetGo` will be generalized to `k`, which is too polymorphic for the RHS `foo` (which expects the return kind of `LetGo` to be `Type`). Now, if `sGo` is a locally defined function, as in comment:7: {{{#!hs sSconcat :: forall x. x sSconcat = undefined where sGo :: x -> Proxy LetGo sGo _ = foo }}} Before the offending commit, then the return kind of `LetGo` was //not// generalized, causing it to default to `Type`, which makes everything go through. After the offending commit, however, GHC now kind-generalizes local definitions, which means that the return kind of `LetGo` is now generalized to `k` again. In other words, `sGo` fails to kind-check for the same reasons that it would fail to kind-check if it were defined at the top level. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15472#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler