I tried to follow the instructions at
https://prime.haskell.org/wiki/Libraries/Proposals/SemigroupMonoid
and I got a <<loop>> when my code runs with ghc 8.2.2.

My code looked like this:

import Data.Monoid
import Prelude

data InfoVal v = NoInfoVal | InfoVal v
        deriving (Show)

instance Monoid (InfoVal v) where
        mempty = NoInfoVal
        mappend _ v@(InfoVal _) = v
        mappend v NoInfoVal = v

Following the "recommended variant" for compatible code, I changed that
too:

{-# LANGUAGE CPP #-}

import Data.Monoid
import qualified Data.Semigroup as Sem
import Prelude

data InfoVal v = NoInfoVal | InfoVal v
        deriving (Show)

instance Monoid (InfoVal v) where
        mempty = NoInfoVal
#if !(MIN_VERSION_base(4,11,0))
        mappend = (<>)
#endif

instance Sem.Semigroup (InfoVal v) where
        _ <> v@(InfoVal _) = v
        v <> NoInfoVal = v

This loops because <> comes from Data.Monoid not from Data.Semigroup,
so mappend = mappend.

Note that I diverged slightly from the instructions to get to this wrong
code; I imported Data.Semigroup qualified. Without the qualification,
the above code fails to compile, with Ambiguous occurrence ‘<>’

So, the transition instructions don't produce code that ghc 8.2.2 can build,
and when the obvious fix is made to get it to compile, it compiles into a
loop, that is *not* a loop when compiled with newer versions of ghc.

--
see shy jo
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