Avoid an overloaded virtual warning.

Use using statements to avoid hiding the overloaded virtual.

The error was found by Clang.
Avoid an overloaded virtual warning.

Remove the default argument and let some subclass signatures match the
ones on the parent class.

The error was found by Clang.
Disable -Woverloaded-virtual for clang.

Clang 3.4 compiles cleanly with this warning enabled, but Clang 3.2
fails. (Clang 3.3 hasn't been tested.)

This reverts 5fb8dc8a3356b284b8890481df62cfb087a2c300