future::error() interface is odd

[sean-parent]

The error() function returns an optional<exception_ptr>. Wouldn't just a, possibly null, exception_ptr be sufficient? (I know we perhaps have two states here, ready, and an exception occurred, perhaps we can collapse that and add a ready() or status() member. The name is also a bit unexpected, I would expect it to be called exception() to match set_exception(). We could add exception() as a non-breaking change and deprecate error(). The current form makes writing recover() extra painful (see issue #263):

f.recover([](auto f) {
    auto e = f.error();
    if (e && *e) //...
}

compared to:

f.recover([](auto f) {
    if (auto e = f.exception()) //...
}

[FelixPetriconi]

What would be the purpose of ready()? We already have an is_ready() function?

[FelixPetriconi]

Fixed in 1.5.0