No .fail without .then?

[pwuertz]

In contrast to the QPromise::reject promise that is being tested in tst_fail, regular non-void promises fail to compile .fail methods being attached directly to p:

auto p = QPromise<int>([&](
    const QPromiseResolve<int>& resolve,
    const QPromiseReject<int>& reject)
{
    resolve(42);
    reject(43);
});
p.fail([](int x) {
    Q_UNUSED(x);
});

...qtpromise/src/qtpromise/qpromise_p.h:604: error: no matching function for call to ‘QtPromisePrivate::PromiseData<int>::resolve()’
             promise->m_d->resolve();
             ^~~~~~~

If .fail is preceded by a dummy .then method, the fail works:

p.then([](){}).fail([](int x) {
    Q_UNUSED(x);
});

[simonbrunel]

That's weird, it should work, maybe broken by fa987a5 or 7b0cba5. Does it work with 0.3?

[simonbrunel]

Actually, fail() is supposed to recover an error and thus must return the "replacement" value required to continue the promise chain:

auto p = QPromise<int>([&](auto resolve, auto reject) {
    reject(43);
});

p.fail([](int x) {
    Q_UNUSED(x);
    return -1;
});

[pwuertz]

Oh, that makes sense. So just a misunderstanding on my part, thanks for clearing that up.

If I happen to find a good spot for a static_assert with a fitting message for such a mistake I'll let you know.