Inconsistent treatment of overlap between type and kind variables in type families

[goldfire]

The following code compiles on 7.7.20121031:

type family Foo (x :: *) (y :: x)
type instance Foo Bool Int = Int

After some poking around, I discovered that GHC is treating the type variable x and the kind variable x as distinct. In core, Foo has three arguments.

The following code does not compile:

type family Bar (x :: *) :: x

The error is

    Type variable `x' used in a kind
    In the kind `x'

I can see arguments in favor of either of the above treatments (separate namespaces for type and kind variables vs. unified namespace), but the current implementation seems inconsistent to me.

[simonpj@…]

commit 661c1c112eb40efd75ac61643c3e1231f60d58d4

Author: Simon Peyton Jones <simonpj@microsoft.com>
Date:   Mon Nov 26 12:18:15 2012 +0000

    Improve error message when a variable is used both as kind and type variable
    
    Fixes Trac #7404

 compiler/rename/RnTypes.lhs |   15 +++++++++++++++
 1 files changed, 15 insertions(+), 0 deletions(-)

[simonpj]

See also #6021.