[GHC] #15791: typeKind confuses Type and Constraint

[GHC]

#15791: typeKind confuses Type and Constraint
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           Reporter:  goldfire       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The `typeKind` function takes a type and reports its kind. It gets this
 wrong, sometimes: if you ask for `typeKind` of the quantified constraint
 `Eq a => C a` (where `C` is a class), you get `Type`. This is wrong,
 though I don't have a concrete case where GHC misfires.

 We ''could'' fix `typeKind` to get this right, but doing so would require
 making the `FunTy` case recursive (right now, it always returns `Type`).
 This is wasteful, because most clients of `typeKind` do not care about the
 distinction between `Type` and `Constraint`.

 Instead, I propose making a new `tcTypeKind` which behaves correctly in
 this case.

 We could also muse about making `tcTypeKind` monadic so that it can look
 through unification variables. This might also simplify the story around
 `Note [The tcType invariant]`.

 Regardless of the "make monadic" idea, I claim that, when this is done, we
 should have `isPredTy` be functionally equivalent to `(== Constraint) .
 tcTypeKind`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15791>
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