https://www.reddit.com/r/haskell/comments/1dsfd7i/comment/lfhqmgi/?context=3

restrict-runSqlT.hs

{-# LANGUAGE DerivingStrategies #-}
{-# LANGUAGE DataKinds #-}

import GHC.TypeLits

newtype SqlT m a = SqlT (m a)
  deriving stock (Functor)
  deriving newtype (Applicative, Monad)

runSqlT :: CanRunSqlT m => SqlT m a -> m a
runSqlT (SqlT ma) = ma

class CanRunSqlT m

instance {-# OVERLAPPABLE #-} CanRunSqlT m

instance TypeError (Text "Cannot runSqlT to SqlT") => CanRunSqlT (SqlT m)

{-
ghci> runSqlT (SqlT (pure ())) :: IO ()
ghci> runSqlT (SqlT (SqlT (pure ()))) :: SqlT (SqlT IO) ()

<interactive>:14:1: error: [GHC-64725]
    • Cannot runSqlT to SqlT
    • In the expression:
          runSqlT (SqlT (SqlT (pure ()))) :: SqlT (SqlT IO) ()
      In an equation for ‘it’:
          it = runSqlT (SqlT (SqlT (pure ()))) :: SqlT (SqlT IO) ()
-}