Consider the following program:

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TemplateHaskell #-}
{-# OPTIONS_GHC -ddump-splices #-}
module Bug where

class Foo1 a where
  bar1 :: forall b. a -> b -> b
  bar1 _ x = (x :: b)

$([d| class Foo2 a where
        bar2 :: forall b. a -> b -> b
        bar2 _ x = (x :: b)
    |])

Foo1 typechecks, so naturally you'd expect Foo2 to typecheck as well. Prepare to be surprised:

$ /opt/ghc/8.2.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )
Bug.hs:(10,3)-(13,6): Splicing declarations
    [d| class Foo2_aoA a_aoC where
          bar2_aoB :: forall b_aoD. a_aoC -> b_aoD -> b_aoD
          bar2_aoB _ x_aoE = (x_aoE :: b_aoD) |]
  ======>
    class Foo2_a3JQ a_a3JS where
      bar2_a3JR :: forall b_a3JT. a_a3JS -> b_a3JT -> b_a3JT
      bar2_a3JR _ x_a3JU = x_a3JU :: b_aoD

Bug.hs:10:3: error:
    • Couldn't match expected type ‘b1’ with actual type ‘b’
      ‘b’ is a rigid type variable bound by
        the type signature for:
          bar2 :: forall b. a0 -> b -> b
        at Bug.hs:(10,3)-(13,6)
      ‘b1’ is a rigid type variable bound by
        an expression type signature:
          forall b1. b1
        at Bug.hs:(10,3)-(13,6)
    • In the expression: x_a3JU :: b
      In an equation for ‘bar2’: bar2 _ x_a3JU = x_a3JU :: b
    • Relevant bindings include
        x_a3JU :: b (bound at Bug.hs:10:3)
        bar2 :: a0 -> b -> b (bound at Bug.hs:10:3)
   |
10 | $([d| class Foo2 a where
   |   ^^^^^^^^^^^^^^^^^^^^^^...

Notice how in the quoted Foo2 declaration, the scoping is correct: b_a0D is used in both the type signature for bar2_a0B as well as in its default implementation. But after splicing, there are now two different bs: the one in the type signature (b_a3JT), and the one in the default implementation (b_aoD)! This causes the resulting type error.

This is a regression that was introduced somewhere between 7.10.3 and 8.0.1, since it works in 7.10.3:

$ /opt/ghc/7.10.3/bin/ghci Bug.hs
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Bug.hs:(10,3)-(13,6): Splicing declarations
    [d| class Foo2_awn a_awp where
          bar2_awo :: forall b_awq. a_awp -> b_awq -> b_awq
          bar2_awo _ x_awr = (x_awr :: b_awq) |]
  ======>
    class Foo2_a3zs a_a3zu where
      bar2_a3zt :: forall b_awq. a_a3zu -> b_awq -> b_awq
      bar2_a3zt _ x_a3zv = x_a3zv :: b_awq
Ok, modules loaded: Bug.

But not in any version of GHC since 8.0.1.