RefBase: document 0 strong count, wp<>::promote

    delete foo;

}

TEST(RefBase, NoStrongCountPromoteFromWeak) {

    bool isDeleted;

    Foo* foo = new Foo(&isDeleted);

    wp<Foo> weakFoo = wp<Foo>(foo);

    EXPECT_FALSE(isDeleted);

    {

        sp<Foo> strongFoo = weakFoo.promote();

        EXPECT_EQ(strongFoo, foo);

    }

    // this shows the justification of wp<>::fromExisting.

    // if you construct a wp<>, for instance in a constructor, and it is

    // accidentally promoted, that promoted sp<> will exclusively own

    // the object. If that happens during the initialization of the

    // object or in this scope, as you can see 'Foo* foo' is unowned,

    // then we are left with a deleted object, and we could not put it

    // into an sp<>.

    //

    // Consider the other implementation, where we disallow promoting

    // a wp<> if there are no strong counts. If we return null, then

    // the object would be unpromotable even though it hasn't been deleted.

    // This is also errorprone.

    //

    // attemptIncStrong aborting in this case is a backwards incompatible

    // change due to frequent use of wp<T>(this) in the constructor.

    EXPECT_TRUE(isDeleted);

}

TEST(RefBase, DoubleOwnershipDeath) {

    bool isDeleted;

    auto foo = sp<Foo>::make(&isDeleted);