Test Trac #2247

testsuite/tests/typecheck/should_fail/T2247.hs

+{-# LANGUAGE FunctionalDependencies, MultiParamTypeClasses, UndecidableInstances, FlexibleInstances #-}
+
+module T2247 where
+
+class FD a b | a -> b
+instance CFD a b => FD a b
+
+class {- FD a b => -} CFD a b
+instance CFD Bool Char
+instance CFD Bool Bool
+
+f' :: FD Bool Bool => Bool
+f' = True
+
+g' :: FD Bool Char => Bool
+g' = False
+
+x = f'

testsuite/tests/typecheck/should_fail/T2247.stderr

+
+T2247.hs:6:10:
+    Illegal instance declaration for `FD a b'
+      Multiple uses of this instance may be inconsistent
+      with the functional dependencies of the class.
+    In the instance declaration for `FD a b'

testsuite/tests/typecheck/should_fail/all.T

@@ -293,3 +293,4 @@ test('T7525', normal, compile_fail, [''])
 test('T7368a', normal, compile_fail, [''])
 test('T7545', normal, compile_fail, [''])
 test('T7279', normal, compile_fail, [''])
+test('T2247', normal, compile_fail, [''])