https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110344
--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
Version of the paper testcase that just adds constexpr, that we currently
reject:
#include <string_view>
struct Sheep {
constexpr std::string_view speak() const noexcept { return "Baaaaaa"; }
};
struct Cow {
constexpr std::string_view speak() const noexcept { return "Mooo"; }
};
class Animal_View {
private:
const void *animal;
std::string_view (*speak_function)(const void *);
public:
template <typename Animal>
constexpr Animal_View(const Animal &a)
: animal{&a}, speak_function{[](const void *object) {
return static_cast<const Animal
*>(object)->speak();
}} {}
constexpr std::string_view speak() const noexcept {
return speak_function(animal);
}
};
// This is the key bit here. This is a single concrete function
// that can take anything that happens to have the "Animal_View"
// interface
constexpr std::string_view do_speak(Animal_View av) { return av.speak(); }
int main() {
// A Cow is a cow. The only think that makes it special
// is that it has a "std::string_view speak() const" member
constexpr Cow cow;
constexpr auto result = do_speak(cow);
return static_cast<int>(result.size());
}
