C++03. Test for rvalue-vs-lvalue at compile-time, not just at runtime

Question

In C++03, Boost\'s Foreach, using this interesting technique, can detect at run-time whether an expression is an lvalue or an rvalue. (I found that via this StackOv

Answer 1

The address-of operator (&) can only be used with an lvalue. So if you used it in an SFINAE test, you could distinguish at compile-time.

A static assertion could look like:

#define STATIC_ASSERT_IS_LVALUE(x) ( (sizeof &(x)), (x) )

A trait version might be:

template
struct has_lvalue_subscript
{
    typedef char yes[1];
    typedef char no[2];

    yes fn( char (*)[sizeof (&(((T*)0)->operator[](0))] );
    no fn(...);
    enum { value = sizeof(fn(0)) == 1 };
};

and could be used like

has_lvalue_subscript< std::vector >::value

(Warning: not tested)

I can't think of any way to test an arbitrary expression valid in the caller's context, without breaking compilation on failure.