[GHC] #13742: Code using ConstraintKinds needs explicit kind signature with GHC 8.2.1

[GHC]

#13742: Code using ConstraintKinds needs explicit kind signature with GHC 8.2.1
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        Reporter:  albertov          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:
       Component:  Compiler (Type    |              Version:  8.2.1-rc2
  checker)                           |             Keywords:
      Resolution:                    |  ConstraintKinds, KindSignatures
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 ezyang):

 So, the proximal cause of the error is that we're kind-checking A, B, C
 and D in one SCC where 8.0 wasn't previously, but one might imagine kind
 inference should still have come up with the correct kinds.  We have a
 problem with the fact that `( A l, A l )` is overloaded: GHC doesn't know
 whether or not this is `Constraint -> Constraint -> Constraint`, or `* ->
 * -> *`.  It decides the latter, and then you get a type error.  Here's a
 relevant note from GHC source code:

 {{{
 Note [Inferring tuple kinds]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Give a tuple type (a,b,c), which the parser labels as
 HsBoxedOrConstraintTuple,
 we try to figure out whether it's a tuple of kind * or Constraint.
   Step 1: look at the expected kind
   Step 2: infer argument kinds

 If after Step 2 it's not clear from the arguments that it's
 Constraint, then it must be *.  Once having decided that we re-check
 the Check the arguments again to give good error messages
 in eg. `(Maybe, Maybe)`

 Note that we will still fail to infer the correct kind in this case:

   type T a = ((a,a), D a)
   type family D :: Constraint -> Constraint

 While kind checking T, we do not yet know the kind of D, so we will
 default the
 kind of T to * -> *. It works if we annotate `a` with kind `Constraint`.
 }}}

 This is a pretty similar situation.

 In the old world order, we first added a provisional kind for `D` to the
 environment before computing the kinds for type synonyms, which is why `A
 l` was correctly kinded.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13742#comment:4>
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