[GHC] #12466: Typechecker regression: Inaccessible code in a type expected by the context

[GHC]

#12466: Typechecker regression: Inaccessible code in a type expected by the context
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      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 propose (comment:31) to report an error for a Given insoluble (e.g. `Int
 ~ Bool`) only if
  * there is an enclosing pattern match
  * that binds some equalities

 Otherwise silently ignore Given insolubles.  That will resolve all the
 complaints here, I think.

 It's possible that we should warn; but it'd have to be not in `-Wall`
 because in some cases (like the instance one above) you can't "fix" the
 program to avoid the warning.  So for now I'm inclined just to silently
 ignore.

 OK?  Patch in preparation.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:34>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler