The following code compiles and runs fine with libfmt8. https://godbolt.org/z/bGGPM1Mrs

#include <array>
#include <fmt/core.h>

template <std::size_t N>
class fixed_string {
public:
    static_assert(N > 0);
    constexpr fixed_string() : m_size(0) {};
    constexpr fixed_string(const char* str) {
        m_size = std::min(strlen(str), N - 1);
        std::copy(str, str + m_size, &m_data[0]);
        m_data[m_size] = 0;
    }
    constexpr auto begin() const noexcept { return &m_data[0]; }
    constexpr auto cbegin() const noexcept { return &m_data[0]; };
    constexpr auto end() const noexcept { return &m_data[m_size]; }
    constexpr auto cend() const noexcept { return &m_data[m_size]; };
    constexpr const char& operator[](std::size_t pos) const { return m_data[pos]; }
    constexpr const char* data() const noexcept { return &m_data[0]; }
    constexpr std::size_t size() const noexcept { return m_size; }
private:
    std::size_t m_size;
    std::array<char, N> m_data;
};

template <std::size_t N>
class fmt::formatter<fixed_string<N>> : fmt::formatter<std::string_view> {};

int main() {
    fixed_string<40> str = "hello";
    fmt::print(">>>{:^10}<<<", str);
}

but starting with libfmt9 it seems this no longer works. https://godbolt.org/z/h41frMo6a