[Haskell-cafe] Is a type synonym declaration really a synonym ?

[alpheccar]

Can someone confirm me that:

type TA = A :+: B
type TB = C :+: D
type T = TA :+: TB

is not equivalent to

type T = A :+: B :+: C :+: D

where I have defined

infixr 6 :+:
data (f :+: g)
data A
data B
data C
data D

I have a computation at type level which is working with the later  
definition of T but not with the former (ghc 6.8.1)

Thanks.