Given the following C++23 code (Godbolt):
<code>template<class R, class T>
concept container_compatible_range =
std::ranges::input_range<R> && std::convertible_to<std::ranges::range_reference_t<R>, T>;
template<class K, class V>
struct Map {
using value_type = std::pair<K, V>;
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_bad(R&& rg) {
for (value_type e : rg) {
// ~~~~
}
}
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_good(R&& rg) {
std::ranges::for_each(rg, [&](value_type e) {
// ~~~~
});
}
};
int main() {
Map<int, int> m;
std::tuple<int, int> a[2] = {}; // for example
m.insert_range_bad(a);
m.insert_range_good(a);
}
</code>
<code>template<class R, class T>
concept container_compatible_range =
std::ranges::input_range<R> && std::convertible_to<std::ranges::range_reference_t<R>, T>;
template<class K, class V>
struct Map {
using value_type = std::pair<K, V>;
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_bad(R&& rg) {
for (value_type e : rg) {
// ~~~~
}
}
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_good(R&& rg) {
std::ranges::for_each(rg, [&](value_type e) {
// ~~~~
});
}
};
int main() {
Map<int, int> m;
std::tuple<int, int> a[2] = {}; // for example
m.insert_range_bad(a);
m.insert_range_good(a);
}
</code>
template<class R, class T>
concept container_compatible_range =
std::ranges::input_range<R> && std::convertible_to<std::ranges::range_reference_t<R>, T>;
template<class K, class V>
struct Map {
using value_type = std::pair<K, V>;
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_bad(R&& rg) {
for (value_type e : rg) {
// ~~~~
}
}
template<class R>
requires container_compatible_range<R, value_type>
void insert_range_good(R&& rg) {
std::ranges::for_each(rg, [&](value_type e) {
// ~~~~
});
}
};
int main() {
Map<int, int> m;
std::tuple<int, int> a[2] = {}; // for example
m.insert_range_bad(a);
m.insert_range_good(a);
}
I’ve been told that there are “contrived corner cases” where insert_range_good will successfully compile, but insert_range_bad will fail (in a SFINAE-unfriendly, hard-error kind of way) and/or Do The Wrong Thing.
What are these corner cases?