[GHC] #16139: GHC confused about type synonym kind with QuantifiedConstraints

[GHC]

#16139: GHC confused about type synonym kind with QuantifiedConstraints
-------------------------------------+-------------------------------------
        Reporter:  Ashley Yakeley    |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
                                     |  QuantifiedConstraints
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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 This is more fall-out from the `Constraint` vs `Type` distinction.  The
 kinding rules are
 {{{
           ty : TYPE rep
           `a` is not free in rep
 (FORALL1) -----------------------
           forall a. ty : TYPE rep

           ty : Constraint
 (FORALL2) -------------------------
           forall a. ty : Constraint
 }}}
 How do we know which of these rules to use?  Currently we do it by looking
 at the "expected kind", using `FORALL2` is the expected kind is
 `Constraint`. That's why it works if you give a kind signature.

 You could say "infer the kind of the body type, and use that to choose";
 but that's vulnerable to exactly what constraints are solved when.

 Or perhaps you could do both.

 It's very uncomfortable having two different rules.  I've often wondered
 about defining
 {{{
 type Constraint = TYPE ConstraintRep
 }}}
 in which case one rule would do.  Richard thinks this won't work but I
 forget why.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16139#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler