std::is_destructible with private destructors

// Is the Standard's definition of std::is_destructible unclear?
#include <cstdio>
#include <type_traits>
#include <utility>

struct PrivateDestructor
{
public:
    void *TestFunction();
    void Destroy() { delete this; }
private:
    ~PrivateDestructor() { }
};

void *PrivateDestructor::TestFunction()
{
    typedef PrivateDestructor U;
    // Expression from the Standard is well-formed (evaluates here to "void")...
    decltype(std::declval<U&>().~U()) *ptr = this;
    return ptr;
}

int main()
{
    // ...yet this is false in GCC and Clang.  Is the Standard unclear?
    std::printf("%d\n", std::is_destructible<PrivateDestructor>::value);

    PrivateDestructor *obj = new PrivateDestructor();
    std::printf("%p\n", obj->TestFunction());
    obj->Destroy();
    return 0;
}