[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`
-------------------------------------+-------------------------------------
        Reporter:  hvr               |                Owner:  simonpj
            Type:  bug               |               Status:  merge
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler (Type    |              Version:  8.0.1-rc2
  checker)                           |
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  GHC rejects       |            Test Case:
  valid program                      |  typecheck/should_compile/T11608
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by simonpj):

 * status:  closed => merge


Comment:

 Here's the patch for the other three
 {{{

 commit 3c29c770be7a8c7268dcb8d8624853428aa42071
 Author: Simon Peyton Jones <simonpj at microsoft.com>
 Date:   Mon Feb 29 14:12:28 2016 +0000

     Do not check synonym RHS for ambiguity

     With this patch we no longer check the RHS of a type synonym
     declaration for ambiguity.  It only affects type synonyms with foralls
     on the RHS (which are rare in the first place), and it's arguably
     over-aggressive to check them for ambiguity.  See TcValidity
     Note [When we don't check for ambiguity]

     This fixes the ASSERT failures in
        th                        T3100
        typecheck/should_compile  T3692
        typecheck/should_fail     T3592
 }}}
 Just about worth merging.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11608#comment:18>
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