[GHC] #9033: Type error suppresses defaulting, causes spurious errors

[GHC]

#9033: Type error suppresses defaulting, causes spurious errors
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        Reporter:  jun0          |            Owner:
            Type:  bug           |           Status:  new
        Priority:  normal        |        Milestone:
       Component:  Compiler      |          Version:  7.8.2
      Resolution:                |         Keywords:  defaulting, ambiguous
Operating System:                |     Architecture:  Unknown/Multiple
  Unknown/Multiple               |       Difficulty:  Unknown
 Type of failure:  None/Unknown  |       Blocked By:
       Test Case:                |  Related Tickets:
        Blocking:                |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"ba2e20149e2addaccf5ce3122d3a6e93da696a0a/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="ba2e20149e2addaccf5ce3122d3a6e93da696a0a"
 Do type-class defaulting even if there are insoluble constraints

 The argument in Trac #9033 is very compelling: we should not report 20
 errors, fix one, and have the other 19 disappear.  They were spurious
 in the first place.

 The fix was easy; do type-class defaulting uncondionally, rather than
 only if there are no insoluble constraints.

 See Note [When to do type-class defaulting] in TcSimplify.

 Error messages generally improve, especially tc211 which actually
 had an example of precisely this phenomenon.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9033#comment:1>
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