How to check if method is deleted and branch based on that?

Question

My program has a meta object that takes a strategy at compile time. The strategy may either implement the method begin() or have it marked as delete'd.

In the implementation, I now want to branch on exactly this condition and roll my own implementation of begin() in the case where the strategy doesn't provide one.

Is this somehow possible?

Here's what I want:

Demo

#include <cstdio>

struct implA
{
    auto begin() { return start; }

    void* start;
};

struct implB
{
    auto begin() = delete;
};

template <typename Impl>
struct meta
{
    auto begin() {
        if constexpr (/* implementation available... */) {
            // ... use it
            return impl_.begin;
        } else {
            // ... roll our own
            return start;
        }
    }

    void* start;

    Impl impl_;
};

int main()
{
    meta<implA> m;
    meta<implB> m;
}