[GHC] #13655: Spurious untouchable type variable in connection with rank-2 type and constraint family

[GHC]

#13655: Spurious untouchable type variable in connection with rank-2 type and
constraint family
-------------------------------------+-------------------------------------
        Reporter:  jeltsch           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1-rc2
  checker)                           |
      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 simonpj):

 I can see that it seems odd.  Imagine that you wrote
 {{{
 f :: (forall b . F a b => T b c) -> a
 f _ = undefined

 g :: (forall bb . F aa bb => T bb cc) -> aa
 g x = f x
 }}}
 You'd expect that to work: the two signatures are the same.   But in the
 call in g's rhs we have:
 {{{
 Expected type of f's arg:              forall b. F a0 b => T b c0)
 Actual type of x (after instantation): T bb0 cc
            plus a "wanted" constraint: F aa bb0
 }}}
 where the "0" things are unification variables.  Of course this is easily
 soluble by unifying `bb0=b, a0=aa, c0=cc`.  The `a0=aa` is enforced by the
 result type of the call to `(f x)` but the other two are not. And the
 "untouchable" bit is because we don't unify under a given equality
 constraint, or something that might be a given equality, like `(F a0 b)`.
 (Why: see the OutsideIn paper.)

 The ambiguity check, which is failing here, is precisely implementing this
 reasoning.

 In short: it's all behaving as designed.  The error message is unhelpful,
 but it IS a subtle issue.

 I wish I could be more helpful.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13655#comment:1>
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