[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:  8.2.1
       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:                    |
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Changes (by RyanGlScott):

 * cc: ekmett (added)


Comment:

 Sorry, my previous estimate was off the mark. It's not that the definition
 of `$dmfoo` is being inlined. Rather, after reading
 d2958bd08a049b61941f078e51809c7e63bc3354 more closely, the real culprit is
 that prior to that commit, default methods weren't being typechecked for
 each instance! As a result, the code was sneaking past the typechecker.

 But I'm still confused as to why defining:

 {{{#!hs
 instance Foo Char where
   foo _ x = x
 }}}

 typechecks without issue (on all versions of GHC), but defining the
 instance in terms of a helper method:

 {{{#!hs
 fooDefault :: (a ~ Int => Int) -> a -> a
 fooDefault _ a2 = a2

 instance Foo Char where
   foo = fooDefault @Char
 }}}

 is rejected with an error.

 cc'ing Edward Kmett on this one, since if `lens` really is using code that
 shouldn't typecheck, `lens` is going to need to come up with a workaround.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12466#comment:13>
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