[GHC] #10177: Typeable solver regression

[GHC]

#10177: Typeable solver regression
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        Reporter:  glguy             |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  highest           |               Milestone:  7.10.1
       Component:  Compiler (Type    |                 Version:  7.10.1-rc3
  checker)                           |                Keywords:
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  GHC rejects       |               Test Case:
  valid program                      |  testsuite/tests/typecheck/should_compile/T10177
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 What about this?

 {{{
 instance (Typeable a, Typeable b) => C (a b)
 }}}

 That looks good to me. But what you have above is just a simple
 substitution on my instance, so I think it should be good.

 Can the `Typeable` solver just immediately decompose `Typeable (V n)` to
 `Typeable n`? Is there a threat that the overlapping instances are
 semantically different?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10177#comment:6>
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