[GHC] #10351: Inferred type is rejected

[GHC]

#10351: Inferred type is rejected
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:  fixed             |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |  typecheck/should_fail/T10351
                                     |                Blocking:
                                     |  Differential Revisions:
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Comment (by simonpj):

 The difficulty before is that GHC would infer a type ''and then complain
 that its own inferred type was illegal''.  That seems confusing.

 Moreover sometimes, if it had simply refrained from generalising over that
 constraint, the program would have been accepted.  So the change means
 that strictly more programs are accepted.

 But as you discovered, failing to generalise over a constraint may mean
 that it is reported as unsolved; and only when you fix that does it ask
 for `FlexibleContexts`.

 I'm open to suggestions, but the new behaviour seems cleaner to me.

 Simon

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10351#comment:4>
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