TRY is greedy

BOOST_OUTCOME_TRYV(expr)/BOOST_OUTCOME_TRY(expr) has ‘greedier’ implicit conversion semantics than basic_result<T, E, NoValuePolicy> . For example, this code won’t compile:

outcome::result<int, std::error_code> test(outcome::result<int, std::errc> r)
{
    return r;  // you need to use explicit construction here
    // i.e. return outcome::result<int>(r);
}

This is chosen because there is a non-trivial conversion between std::errc and std::error_code, so even though that conversion is implicit for std::error_code, Outcome does not expose the implicitness here in order to keep the implicit constructor count low (implicit constructors add significantly to build times).

The TRY operation is more greedy though:

outcome::result<int, std::error_code> test(outcome::result<int, std::errc> r)
{
    BOOST_OUTCOME_TRY(r);    // no explicit conversion needed
    return r.value();
}

This is because result<int, std::error_code> will implicitly construct from anything which either int or std::error_code will implicitly construct from. However, result<int, std::error_code> will not implicitly construct from result<int, std::errc>.

Thus bear this in mind during usage: TRY is greedier for implicit conversions than the Outcome types themselves.