Add a test for #2846

testsuite/tests/ghc-regress/typecheck/should_fail/T2846b.hs

+{-# LANGUAGE ImpredicativeTypes, FlexibleContexts #-}
+module T2846 where
+
+f :: String
+f = show ([1,2,3] :: [Num a => a])
+

testsuite/tests/ghc-regress/typecheck/should_fail/T2846b.stderr

+
+T2846b.hs:5:5:
+    No instance for (Show (Num a => a))
+      arising from a use of `show'
+    Possible fix: add an instance declaration for (Show (Num a => a))
+    In the expression: show ([1, 2, 3] :: [Num a => a])
+    In an equation for `f': f = show ([1, 2, 3] :: [Num a => a])

testsuite/tests/ghc-regress/typecheck/should_fail/all.T

@@ -227,3 +227,4 @@ test('IPFail', normal, compile_fail, [''])
 
 test('T3468', extra_clean(['T3468.hi-boot']), 
      multimod_compile_fail, ['T3468', '-v0'])
+test('T2846b', normal, compile_fail, [''])