[GHC] #11948: GHC forgets constraints

[GHC]

#11948: GHC forgets constraints
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        Reporter:  crockeea          |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.2
       Component:  Compiler          |              Version:  7.10.3
      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:"9421b0c77122d40bf72665ea9f90dca64a0a0ae2/ghc" 9421b0c/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="9421b0c77122d40bf72665ea9f90dca64a0a0ae2"
 Warn about simplifiable class constraints

 Provoked by Trac #11948, this patch adds a new warning to GHC

   -Wsimplifiable-class-constraints

 It warns if you write a class constraint in a type signature that
 can be simplified by an existing instance declaration.  Almost always
 this means you should simplify it right now; type inference is very
 fragile without it, as #11948 shows.

 I've put the warning as on-by-default, but I suppose that if there are
 howls of protest we can move it out (as happened for -Wredundant-
 constraints.

 It actually found an example of an over-complicated context in CmmNode.

 Quite a few tests use these weird contexts to trigger something else,
 so I had to suppress the warning in those.

 The 'haskeline' library has a few occurrences of the warning (which
 I think should be fixed), so I switched it off for that library in
 warnings.mk.

 The warning itself is done in TcValidity.check_class_pred.

 HOWEVER, when type inference fails we get a type error; and the error
 suppresses the (informative) warning.  So as things stand, the warning
 only happens when it doesn't cause a problem.  Not sure what to do
 about this, but this patch takes us forward, I think.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11948#comment:3>
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