CWG3071 [dcl.struct.bind] Negative `tuple_size` in structured binding.

[cor3ntin]

We should specify that a tuple needs to decompose into 0 or more elements. It is unclear what would happen otherwise, ie in this example

namespace std {
template <typename T> struct tuple_size;
}

struct S {
    int a;
};

template <> struct std::tuple_size<S> {
    static constexpr int value = -1;
};

void f(auto) { auto [... a] = S{}; }

This was initially reported as a bug in clang llvm/llvm-project#159563

Proposed change:
In [dcl.struct.bind],

Otherwise, if the qualified-id std​::​tuple_size<E> names a complete class type with a member named value, the expression std​::​tuple_size<E>​::​value shall be a well-formed non-negative integral constant expression and the structured binding size of E is equal to the value of that expression.

[jensmaurer]

CWG3071