#15558: Locally handling an empty constraint -------------------------------------+------------------------------------- Reporter: Ericson2314 | Owner: (none) Type: task | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by RyanGlScott): You're right, there's definitely something fishy going on with this warning. Here's a simplified example: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE RankNTypes #-} module Bug where data T a where MkT :: T Int data Foo a = MkFoo ((a ~ Bool) => ()) f :: T a -> Foo a f MkT = MkFoo () g :: Foo Int g = f MkT }}} {{{ $ /opt/ghc/8.6.1/bin/ghci Bug.hs GHCi, version 8.6.0.20180810: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Bug.hs:11:3: warning: [-Winaccessible-code] • Couldn't match type ‘Int’ with ‘Bool’ Inaccessible code in a pattern with constructor: MkT :: T Int, in an equation for ‘f’ • In the pattern: MkT In an equation for ‘f’: f MkT = MkFoo () | 11 | f MkT = MkFoo () | ^^^ }}} While there //is// inaccessible code here, the warning is highlighting the wrong spot: it should be the argument to `MkFoo`, not the argument to `f`. (The fact that `g` typechecks at all is a testament to this fact.) I suspect that we're using the wrong `SrcSpan` somewhere in GHC, which causes the reported location to be incorrect. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15558#comment:8 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler