The function signature of std::apply does not constrain the template parameter Tuple to be a specialization of std::tuple, so it can still accept a tuple-like objects that defines std::tuple_size_v (godbolt):
#include <tuple>
#include <utility>
#include <array>
int main() {
std::apply([](int, int) {}, std::array{0, 0});
std::apply([](int, int) {}, std::pair {0, 0});
std::apply([](int, int) {}, std::tuple{0, 0});
}
But the description of std::apply in [tuple.apply] is:
20.5.5 Calling a function with a
tupleof arguments
Does this mean that applying std::apply to objects other than std::tuple is undefined behavior?
I highly doubt that the section titles are normative.
The actual function is described as being equivalent to the reference implementation, which uses
getandtuple_size_vto inspect the "tuple" parameter.Cppreference concurs.