Correctness bugs: set::min()/max() ignore comparator; non-const set::operator[] does not compile

[jkalias]

Summary

Two correctness defects in include/set.h. Both are currently masked by the test suite (one via a compiler-conditional expectation, one because the buggy overload is never instantiated) rather than fixed.

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1. set::min() / max() ignore the set's comparator (and are O(n))

Location: include/set.h:607-635

[[nodiscard]] fcpp::optional_t<TKey> min() const
{
    const auto& it = std::min_element(begin(), end());
    if (it != end()) {
        return *it;
    }
    return fcpp::optional_t<TKey>();
}

std::min_element / std::max_element compare elements with operator< on TKey, not with the set's TCompare.

Consequences

• Wrong result for any set instantiated with a custom comparator. The set is stored ordered by TCompare, but min()/max() re-rank by operator<, so they can return the wrong element.
• Fails to compile for key types that provide TCompare but not operator<.
• O(n) where it should be O(1): a std::set is already ordered, so the minimum is *begin() and the maximum is *std::prev(end()).

Evidence — the test papers over this instead of fixing it (tests/set_test.cc:289):

TEST(SetTest, MinCustomType)
{
    const set<person, person_comparator> persons({
        person(15, "Jake"), person(18, "Jannet"),
        person(25, "Kate"), person(62, "Bob")
    });
    const auto minimum = persons.min();
#if defined(__clang__)
    EXPECT_EQ(person(18, "Jannet"), minimum.value());
#else
    EXPECT_EQ(person(62, "Bob"), minimum.value());   // different answer per compiler
#endif
}

The fact that the expected value differs between clang and gcc is a direct symptom of the bug: the answer depends on operator< and iteration order rather than the set's actual ordering.

Suggested fix

[[nodiscard]] fcpp::optional_t<TKey> min() const
{
    if (m_set.empty()) {
        return fcpp::optional_t<TKey>();
    }
    return *begin();              // ordered by TCompare, O(1)
}

[[nodiscard]] fcpp::optional_t<TKey> max() const
{
    if (m_set.empty()) {
        return fcpp::optional_t<TKey>();
    }
    return *std::prev(end());     // O(1)
}

This is deterministic, respects the custom comparator, is O(1), and allows the #if defined(__clang__) branch in SetTest.MinCustomType (and the analogous max test) to be removed.

---

2. Non-const set::operator[] does not compile

Location: include/set.h:1102-1116

TKey operator[](size_t index)
{
    assert_smaller_size(index);
#ifdef CPP17_AVAILABLE
    auto it = std::advance(begin(), index);   // std::advance returns void
    return *it;
#else
    ...
#endif
}

std::advance returns void, so auto it = std::advance(...) is ill-formed. The const overload (set.h:1121-1136) does it correctly:

auto it = begin();
std::advance(it, index);
return *it;

Why it's currently hidden: every call site in the tests uses operator[] on a const set& (e.g. test_contents(const set<int>& set) at tests/set_test.cc:37), so the non-const overload is never instantiated. Any use such as:

fcpp::set<int> s({1, 2, 3});
auto x = s[0];   // selects the non-const overload -> compile error

would fail to build under C++17.

Suggested fix: mirror the const overload:

TKey operator[](size_t index)
{
    assert_smaller_size(index);
    auto it = begin();
    std::advance(it, index);   // works for both C++11 and C++17 paths
    return *it;
}

A regression test that subscripts a non-const set would prevent this from regressing.