Specializing formatting for pointer to class - fmt

Question

I need to provide format-magic to a class hierarchy and usually those things are passed around via pointer. This example here works currently on master:
#include <type_traits>
#include <sstream>
#include <fmt/format.h>
#include <fmt/ostream.h>
struct A {
virtual ~A() {}
virtual std::string name() const { return "A"; }
};
struct B : A {
virtual std::string name() const { return "B"; }
};
template <typename T>
struct fmt::formatter<T, std::enable_if_t<std::is_base_of<A, std::remove_const_t<std::remove_pointer_t<T>>>::value, char>> : fmt::formatter<std::string> {
template <typename FormatCtx>
auto format(const A* a, FormatCtx& ctx) {
return fmt::formatter<std::string>::format(a->name(), ctx);
}
};
template<class... T>
[[noreturn]] void err(fmt::format_string<T...> s, T&&... args) {
throw std::logic_error(fmt::format(s, std::forward<T>(args)...));
}
int main() {
A* a = new A();
B* b = new B();
const A* x = new A();
const B* y = new B();
fmt::print("{}, {}, {}, {}\n", a, b, x, y);
std::ostringstream oss;
fmt::print(oss, "{}\n", a);
err("{}, {}, {}, {}\n", a, b, x, y);
}
However, when I'm going to the latest release 8.1.1, I'm getting:
error: static assertion failed: Formatting of non-void pointers is disallowed.
Now my question is: What's the plan here? Can I do this in future versions of fmt or is this more of an accident?

Answer 1

This is definitely a regression. Supporting a hierarchy of classes can be done as documented in https://fmt.dev/latest/api.html#formatting-user-defined-types using references instead of pointers:
#include <type_traits>
#include <fmt/format.h>
struct A {
virtual ~A() {}
virtual std::string name() const { return "A"; }
};
struct B : A {
virtual std::string name() const { return "B"; }
};
template <typename T>
struct fmt::formatter<T, std::enable_if_t<std::is_base_of<A, T>::value, char>> :
fmt::formatter<std::string> {
template <typename FormatCtx>
auto format(const A& a, FormatCtx& ctx) {
return fmt::formatter<std::string>::format(a.name(), ctx);
}
};
int main() {
B b;
A& a = b;
fmt::print("{}", a); // prints "B"
}