[GHC] #11608: Possible type-checker regression in GHC 8.0 when compiling `microlens`

[GHC]

#11608: Possible type-checker regression in GHC 8.0 when compiling `microlens`
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        Reporter:  hvr               |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  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 Simon Peyton Jones <simonpj@…>):

 In [changeset:"7496be5c0ab96bcc9ab70ab873aa561674b7789d/ghc"
 7496be5c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="7496be5c0ab96bcc9ab70ab873aa561674b7789d"
 Exclude TyVars from the constraint solver

 There is a general invariant that the constraint solver doesn't see
 TyVars, only TcTyVars.  But when checking the generic-default
 signature of a class, we called checkValidType on the generic-default
 type, which had the class TyVar free. That in turn meant that it wasn't
 considered during flattening, which led to the error reported in
 Trac #11608.

 The fix is simple: call checkValidType on the /closed/ type. Easy.

 While I was at it, I added a bunch of ASSERTs about the TcTyVar
 invariant.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11608#comment:10>
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