The following program compiles without issue on GHC 7.6.3 through GHC 7.10.3, but fails to compile on GHC 8.0.1 and GHC HEAD. (I added a CPP guard simply because the DataD constructor changed between 7.10 and 8.0.)

{-# LANGUAGE CPP #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Regression where

import Language.Haskell.TH

data T k
class C a

$(do TyConI (DataD [] tName [ KindedTV kName kKind]
#if __GLASGOW_HASKELL__ >= 800
                   _
#endif
                   _ _) <- reify ''T
     d <- instanceD (cxt []) (conT ''C `appT` (conT tName `appT` sigT (varT kName) kKind)) []
     return [d])

$ /opt/ghc/8.0.1/bin/ghc Regression.hs 
[1 of 1] Compiling Regression       ( Regression.hs, Regression.o )
Regression.hs:(13,3)-(19,15): Splicing declarations
    do { TyConI (DataD []
                       tName_a2RT
                       [KindedTV kName_a2RU kKind_a2RV]
                       _
                       _
                       _) <- reify ''T;
         d_a31Y <- instanceD
                     (cxt [])
                     (conT ''C
                      `appT` (conT tName_a2RT `appT` sigT (varT kName_a2RU) kKind_a2RV))
                     [];
         return [d_a31Y] }
  ======>
    instance C (T (k_avB :: k_avC))

Regression.hs:13:3: error:
    Variable ‘k_avB’ used as both a kind and a type
    Did you intend to use TypeInType?

Somewhat confusingly, you can use instance C (T (k_avB :: k_avC)) on its own, and it will compile without issue. Also, quoting it doesn't seem to trip up this bug, as this also compiles without issue:

{-# LANGUAGE CPP #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module WorksSomehow where

import Language.Haskell.TH

data T k
class C a

$([d| instance C (T k) |])

The original program has several workarounds:

1. Turn off PolyKinds (OK, this isn't a very good workaround...)
2. Add an explicit kind signature to T, e.g., data T (k :: k1)
3. Change the type variable of T to some other letter, e.g., data T p or data T k1

Given that (3) is a successful workaround, I strongly suspect that GHC is confusing the type variable k (which gets ddump-spliced as k_avB) with the kind variable k (which gets ddump-spliced as k_avC) due to their common prefix k.