[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
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        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.2
       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:                    |
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Comment (by simonpj):

 Currently
 {{{
 foo :: (Char ~ Int => Int) -> a -> a
 foo _ a2 = a2
 }}}
 gives a similar message.  Is that reasonable?  The argument to `foo` can't
 be called.

 Instance decls are a bit different because you can change the type
 signature.  Eg here is a simpler example
 {{{
 class C a where
   foo :: (a~Int) => a -> a

 instance C Bool where
   foo = undefined
 }}}
 The instance decl is rejected, by both GHC 7.10 and HEAD.  But of course
 there's nothing you can do about it.  `foo` can't be called, but it needs
 something for the method `foo`.  (It works when you leave out the
 definition of `foo` altogether, but I think only by accident.)

 What we need is NOT to complain about inconsistent 'givens' when
 typechecking instance methods.  And certainly not to fail.

 Would that serve?  It would sadly mean that inaccessible code somewhere
 deep in the body of an instance method wouldn't be reported, but maybe
 that's a lesser evil.

 I have not followed all this stuff about `Conjoined`.  Is it all the same
 question?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:22>
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