add a test for :seti

tests/ghci/scripts/all.T

@@ -81,6 +81,8 @@ test('ghci056',
      ],
      ghci_script, ['ghci056.script'])
 
+test('ghci057', normal, ghci_script, ['ghci057.script'])
+
 test('2452', normal, ghci_script, ['2452.script'])
 test('T2766', normal, ghci_script, ['T2766.script'])
 

tests/ghci/scripts/ghci057.hs

+module Test where
+data T a where
+  C :: T Int
+

tests/ghci/scripts/ghci057.script

+: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
+

tests/ghci/scripts/ghci057.stdout

+options currently set: none.
+base language is: Haskell2010
+with the following modifiers:
+  -XNoDatatypeContexts
+  -XNondecreasingIndentation
+GHCi-specific dynamic flag settings:
+other dynamic, non-language, flag settings:
+  -fno-ghci-history
+  -fimplicit-import-qualified
+warning settings:
+Should fail, GADTs is not enabled
+options currently set: none.
+base language is: Haskell2010
+with the following modifiers:
+  -XGADTs
+  -XGADTSyntax
+  -XNoDatatypeContexts
+  -XNondecreasingIndentation
+  -XMonoLocalBinds
+GHCi-specific dynamic flag settings:
+other dynamic, non-language, flag settings:
+  -fno-ghci-history
+  -fimplicit-import-qualified
+warning settings:
+Should work, GADTs is in force from :set
+options currently set: none.
+base language is: Haskell2010
+with the following modifiers:
+  -XGADTSyntax
+  -XNoDatatypeContexts
+  -XNondecreasingIndentation
+  -XMonoLocalBinds
+GHCi-specific dynamic flag settings:
+other dynamic, non-language, flag settings:
+  -fno-ghci-history
+  -fimplicit-import-qualified
+warning settings:
+Should fail, GADTs is now disabled
+base language is: Haskell2010
+with the following modifiers:
+  -XGADTs
+  -XGADTSyntax
+  -XNoDatatypeContexts
+  -XNondecreasingIndentation
+  -XMonoLocalBinds
+  -XExtendedDefaultRules
+GHCi-specific dynamic flag settings:
+other dynamic, non-language, flag settings:
+  -fno-ghci-history
+  -fimplicit-import-qualified
+warning settings:
+Should fail, GADTs is only enabled at the prompt
+C :: T Int