[GHC] #7854: Constrained method type accepted in Haskell 98 mode

[GHC]

#7854: Constrained method type accepted in Haskell 98 mode
-------------------------------------+-------------------------------------
        Reporter:  refold            |                   Owner:  thomie
            Type:  bug               |                  Status:  patch
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler (Type    |                 Version:  7.6.3
  checker)                           |                Keywords:  newcomer
      Resolution:                    |            Architecture:
Operating System:  Unknown/Multiple  |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |  typechecker/should_fail/T7854
 Related Tickets:  #10118, #10119    |                Blocking:
                                     |  Differential Revisions:  Phab:D688
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Thomie, thanks for working on this.  Inspired by your patches I can see a
 simpler way to achieve the same goal, so I'll do that.

 Specifically I plan to do this:

  * Fix the original bug simply by always doing a `checkValidType` on the
 entire selector-id type. That includes an ambiguity check which is
 precisely what we need.

  * Implement the check when `ConstrainedClassMethods` is ''off'' as a
 special check that does nothing else.

  * As comment:4 acknowledges, GHC has not been making the claimed check
 for ages.  Rather than making `ConstrainedClassMethods` on by default (as
 you propose), I plan to make it implied by `MultiParamTypeClasses`, which
 means that we are out of Haskell-98 land anyway.  This means that some
 programs may break, but (a) the error message suggests adding
 `ConstrainedClassMethods` (b) the fix is easy.  If necessary (e.g. on a
 Stackage run) we can make it on-by-default in 7.10 to ease transition.

 Commit coming.  Thanks for unblocking this.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7854#comment:11>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler