[GHC] #10875: Unexpected defaulting of partial type signatures and inconsistent behaviour when -fdefer-typed-holes is set.

[GHC]

#10875: Unexpected defaulting of partial type signatures and inconsistent behaviour
when -fdefer-typed-holes is set.
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        Reporter:  holzensp          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:
                                     |  PartialTypeSignatures TypedHoles
Operating System:  MacOS X           |            Architecture:  x86_64
 Type of failure:  Incorrect         |  (amd64)
  warning at compile-time            |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by simonpj):

 Interesting examples, thank you!

 As it happens I am in mid-flight on a significant refactoring of the
 wildcard implementation, so I'd like to park these examples and come back
 to them when I commit. It has turned out to be a much bigger job than I
 anticipated, I'm afraid.

 Just on the narrow question of defaulting, I think it's not unreasonable
 that wildcards don't affect type-class defaulting (i.e. in ambiguous
 cases, choose a particular type to fix the type, see
 [https://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-750004.3
 4.3.4 in the language definition].  Wouldn't people complain if adding
 wildcards changed the typing rules?  Can you specify ''precisely'' what
 the new defaulting rules would then be?

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10875#comment:3>
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