[GHC] #16114: strange "instance .. => .. => .. where ..."

[GHC]

#16114: strange "instance .. => .. => .. where ..."
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        Reporter:  j.waldmann        |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > Notice that rnLHsInstType no longer errors if given malformed input!

 I can't for the life of me figure out WHY we made that change.  There is
 only one call to `rnLHsInstType`, and it makes sense

 * For `rnLHsInstType` to error on a malformed instance
 * To use the same function `getLHsInstDeclClass_maybe` as is already used
 for this purpose in `RnNames.getLocalNonValBinders`

 We need to discover the class in the renamer so that we have the right
 "parent" for lookups.

 So can we just put the check back inot `rnLHsInstType`?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16114#comment:3>
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