Compiler: Improved handling of argument default values. For improved backward compatibility (and DWIM) allow 0 (in addition to UNDEFINED) to select the default value when 0 is not a valid value for the argument. This improves backward compatibility where eg code like mixed foo(string|void bar) { if (!bar) bar = "bar"; [...] } has been replaced with mixed foo(string bar = "bar") { [...] } Improves #10086.