[GHC] #15558: Locally handling an empty constraint

[GHC]

#15558: Locally handling an empty constraint
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        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:                    |
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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>
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