#15276: Bogus type in typechecker error recovery -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If the typechecker sees {{{ let f = <rhs> in <body> }}} and there's a type error in `<rhs>`, GHC recovers from the error, binds `f` to "a type that should cause no more trouble", and continues with `<both>` in the hope of finding more type errors. What is "a type that should cause no more trouble"? Well `forall a.a` seems like a good candidate. But, in this commit {{{ commit 6746549772c5cc0ac66c0fce562f297f4d4b80a2 Author: Richard Eisenberg <eir@cis.upenn.edu> Date: Fri Dec 11 18:19:53 2015 -0500 Add kind equalities to GHC. }}} we made the type look like this: `forall r. forall (a :: TYPE r). a` Alas! That type is ill-formed because the kind `TYPE r` escapes the scope of `f`. I discovered this when beefing up `typeKind` in pursuit of #14939 -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15276> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler