Is conversion from result<T> to result<U> a valid use-case?

[ytimenkov]

Working with generic code it may be beneficial if result<T> can be implicitly converted to result<U> if such conversion is allowed for underlying T and U.

For example:

void f(result<std::size_t> ec)
{
}

template<typename T>
void g(T&& arg)
{
    f(result<std::decay_t<T>>{arg});
}

int main()
{
    g(0);

    return 0;
}

results in error

error: could not convert 'boost::system::result<int, boost::system::error_code>((* & arg))' from 'result<int,[...]>' to 'result<long unsigned int,[...]>'

This mostly needed for generic code, where it is impossible (or at least very hard) which type the final function really accepts and argument (or result) may be passed through multiple forwarding functions.

[pdimov]

The converting constructor is implemented on the develop branch now.

[pdimov]

Has been merged to master, should be in 1.80.