[GHC] #15334: (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x)

[GHC]

#15334: (forall x. c x, forall x. d x) is not equivalent to forall x. (c x, d x)
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        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.5
  checker)                           |             Keywords:
      Resolution:                    |  QuantifiedConstraints
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:"fd0f0334189c0c5c9b186bd1b009f706d3d86086/ghc"
 fd0f0334/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="fd0f0334189c0c5c9b186bd1b009f706d3d86086"
 More refactoring in TcValidity

 This patch responds to Trac #15334 by making it an error to
 write an instance declaration for a tuple constraint like
 (Eq [a], Show [a]).

 I then discovered that instance validity checking was
 scattered betweeen TcInstDcls and TcValidity, so I took
 the time to bring it all together, into
   TcValidity.checkValidInstHead

 In doing so I discovered that there are lot of special
 cases.   I have not changed them, but at least they are
 all laid out clearly now.
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15334#comment:10>
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