Where does def_cast work? Regarding the issue of execute_cast compilation failure.

[MannixYang]

In operator.h，execute_cast may fail to compile because no constructor B(expr).

#define PYBIND11_BINARY_OPERATOR(id, rid, op, expr)                                               \
    template <typename B, typename L, typename R>                                                 \
    struct op_impl<op_##id, op_l, B, L, R> {                                                      \
        static char const *name() { return "__" #id "__"; }                                       \
        static auto execute(const L &l, const R &r) -> decltype(expr) { return (expr); }          \
        static B execute_cast(const L &l, const R &r) { return B(expr); }                         \
    };     
 // ...

Can we use if constexpr to avoid compilation failures？Like this:

  static auto execute_cast(const L &l, const R &r){
    if constexpr (!std::is_abstract_v<B> && std::is_constructible_v<B, decltype(expr)>)
        return B(expr);
    else
        throw std::runtime_error("execute_cast is not available"); // maybe return execute(l, r)
  }
};

Similar issue: #1487