GHC 8.0 accepts recursive kind signature that GHC 7.10 rejects

GHC 7.10 rejects this datatype:

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
module Bug where

data T (a :: a)

$ /opt/ghc/7.10.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:5:1:
    Kind variable also used as type variable: ‘a’
    In the data type declaration for ‘T’

But GHC 8.0 accepts the above code! It appears that GHC implicitly freshens the kind-level a so as to have a different name, since the following modification to T:

data T a (a :: a)

Results in this error:

$ /opt/ghc/8.0.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:5:8: error:
    Conflicting definitions for ‘a’
    Bound at: Bug.hs:5:8
              Bug.hs:5:11

Bug.hs:5:16: error:
    Type variable ‘a’ used in a kind.
    Did you mean to use TypeInType?
    the data type declaration for ‘T’

But with this modification:

data T (a :: a) a

You won't get an error message about a being used as both a kind and type:

$ /opt/ghc/8.0.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:5:9: error:
    Conflicting definitions for ‘a’
    Bound at: Bug.hs:5:9
              Bug.hs:5:17

To make things even more confusing, this behavior doesn't seem to carry over to typeclass instances. For example: GHC 8.0 will (rightfully) reject this code:

{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
module Bug where

data T (a :: a)

class C x
instance C (T (a :: a))

$ /opt/ghc/8.0.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:8:16: error:
    Variable ‘a’ used as both a kind and a type
    Did you intend to use TypeInType?

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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Architecture