Test Trac #5957

testsuite/tests/typecheck/should_fail/T5957.hs

+module T5957 where
+
+flex :: Int -> Show a => a -> String
+flex i a = show a ++ show i

testsuite/tests/typecheck/should_fail/T5957.stderr

+
+T5957.hs:3:9:
+    Illegal polymorphic or qualified type: Show a => a -> String
+    Perhaps you intended to use -XRankNTypes or -XRank2Types
+    In the type signature for `flex':
+      flex :: Int -> Show a => a -> String

testsuite/tests/typecheck/should_fail/all.T

@@ -271,3 +271,4 @@ test('T5573b', normal, compile_fail, [''])
 test('T5689', normal, compile_fail, [''])
 test('T5684', normal, compile_fail, [''])
 test('T5858', normal, compile_fail, [''])
+test('T5957', normal, compile_fail, [''])