Fix right this check

This is a supplemental fix for [dmd/pull/1687](https://github.com/D-Programming-Language/dmd/pull/1687).

- std.typecons:

    `Tuple.defaultInit` will create member function template, so it cannot be called in some context. Instead, import `std.traits` and use `std.traits.defaultInit`.

- std.datetime:

    `date` is a method that returns `Date` object. But we cannot call it from `unittest`, because unittest is not a member function.