Function make_all_promise can not deduce template parameters because on some compilers (GCC for example) iterator for std::array is just a pointer type and has not value_type type.

promise p;
p.then([](int a) {
return 1;
}.then(int b) {
return 2;
});

p.resolve(3);

|then| member function actully calls |attach|, and push lambda/callback into std::vector<>.
the quesion is when p invokes the first |then|, and own this callback and states, however, the second |then| will new a promise, and nobody owns this callback.

so when i try to call p.resolve(3), which only trigger the first calback execution ?

Method
const T& promise:: get_or_default(const T& def) const
may return reference to the local object, that will be destroyed after function call.

What is the difference in purpose, it seems you can do similar things with both?

By the way: awesome library, really nice!!

function impl::make_tuple_promise_impl calls std::make_tuple which result is discarded.

MSVC compiler warning:
promise.hpp(1273,22): warning C4834: discarding return value of function with 'nodiscard' attribute