this pointer in lambda-capture by reference

struct CL
{
    int i;
    void fnc()
    {
        [&this](){i=1;};  // (1) error
        [&](){i=1;};      // (2) ok
    }
};

Here is 1st case which doesn't work, but second case does. Why is that?

I see Standard 5.1.2/1:

capture:

identifier

& identifier

this

(i.e. &this doesn't exist)

And 5.1.2/15:

An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy

In second case "this" doesn't captured by copy according 5.1.2/14:

An entity is captured by copy if it is implicitly captured and the capture-default is = or if it is explicitly captured with a capture that does not include an &

But how can I capture "this" pointer by value explicitly? Or is it possible only implicitly with default-capture & ?