[GHC] #11376: Inconsistent specified type variables among functions and datatypes/classes when using -XTypeApplications

[GHC]

#11376: Inconsistent specified type variables among functions and datatypes/classes
when using -XTypeApplications
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        Reporter:  RyanGlScott       |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |             Keywords:
      Resolution:                    |  TypeApplications
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Other             |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by goldfire):

 You're talking about `Note [Instantiate when inferring a type]` in
 !TcBinds. Yes. When we infer a type, we instantiate. The Note has the
 details, but this design makes it so that values with inferred types never
 have any variables available for type application. This is good.

 With `:type`, though, we don't want to instantiate, because doing so might
 change specified variables to invisible ones. Or in cases like `forall a.
 (a ~ Int) => ...`, instantiating and then solving will actually get rid of
 a variable. Instantiating ''when there are no more specified variables''
 isn't ruinous, but I'm sure implementing the special case will cause
 confusion.

 Given that this is, essentially, a pretty-printing issue, maybe it should
 be controlled by `-fprint-explicit-foralls`? With the flag set, we never
 instantiate. Otherwise, we always instantiate.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11376#comment:19>
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