What does P::************ mean in Boost assert.hpp file?

In boost/mpl/assert.hpp, I saw something like this:

template<class Pred>
struct eval_assert {
    typedef typename extract_assert_pred<Pred>::type P;
    typedef typename P::type p_type;
    typedef typename ::boost::mpl::if_c<p_type::value,
        AUX778076_ASSERT_ARG(assert<false>),
        failed ************ P::************
    >::type type;
};

If the first ************ can be treated as pointers of struct failed, the P::************ really doesn't make any sense to me. Is this standard C++?

The point of this code is to help the compiler produce "visible" error messages.

In pre static_assert era, compiling a template-heavy code could easily produce ~100 lines of error messages even for a single mistake, and 99% of those lines are often meaningless.

The 10 pointers trick is useful to point out the actual error, for example:

 BOOST_STATIC_ASSERT((std::is_same<T,U>));

With T=void* and U=char* compiled with gcc produces ~10 error lines, but you can easily see the relevant one:

error: no matching function for call to ‘assertion_failed(mpl_::failed************ std::is_same<void*, char*>::************)’

It's a pointer-to-pointer-to-...-member of type P, where the member is a data member of type pointer-to-pointer-to-...-failed.

In this case the goal is simply to cause compilation to fail by referring to a member of P with a very high degree of probability that it won't exist. In C++11 you'd just use static_assert instead, but of course Boost needs to be portable to pre-C++11 dialects.

F P::* is a "pointer to member of P of type F".

F P::** is a "pointer to pointer to member of P of type F".

More *s adds more "pointer to" in front.

In this case, F is failed ************, i.e., "pointer to pointer to ... pointer to failed".