gPut depends on the nesting structure of :+:

[Kritzefitz]

The documentation of GHC.Generics says this:

However, users /should not rely on a specific nesting strategy/ for :+: and :*: being used.

From reading the code it seems to me as if the code depends on the structure being balanced.

I was able to construct the following example, which breaks the logic of gPut by nesting all constructor to the left:

{-# LANGUAGE DataKinds, TypeOperators #-}
import Data.Serialize
import GHC.Generics

type MyGeneric a = D1 ('MetaData "Foo" "Ghci2" "interactive" 'False) (((C1 ('MetaCons "Foo1" 'PrefixI 'False) U1 :+: C1 ('MetaCons "Foo2" 'PrefixI 'False) U1) :+: C1 ('MetaCons "Foo3" 'PrefixI 'False) U1) :+: C1 ('MetaCons "Foo1" 'PrefixI 'False) U1) a

x :: MyGeneric a
x = M1 $ L1 $ L1 $ L1 $ M1 U1

y :: MyGeneric a
y = M1 $ L1 $ L1 $ R1 $ M1 U1

test :: Either String (Bool, Bool)
test = (\z -> (z == x, z == y)) <$> runGet gGet (runPut $ gPut x)

main :: IO ()
main = print test

When I run the code it outputs Right (False,True), which shows that the encoding and decoding of x isn't equal to x itself, but equal to y.