:set

putStrLn "Should fail, GADTs is not enabled"
data T a where C :: T Int

:set -XGADTs
:set

putStrLn "Should work, GADTs is in force from :set"
:load ghci057.hs

:set -XNoGADTs
:set

putStrLn "Should fail, GADTs is now disabled"
:load ghci057.hs

:seti -XGADTs
:seti

putStrLn "Should fail, GADTs is only enabled at the prompt"
:load ghci057.hs

-- Should work
data T a where C :: T Int
:t C