I have a struct defined inside of a template class and I would like to create a std::formatter for that struct to be able to print that struct instances by using std::print.

The class I have:

template <typename ValueType, std::size_t SizeOfEachStack> requires (SizeOfEachStack > 0)
class ThreeStack
{
public:
    struct StackProperties
    {
        std::size_t min_index{0};
        std::size_t max_index{0};
        std::size_t current_index{0};
    };

    ThreeStack()
    {
        for(std::size_t stack_number = 0; stack_number < num_of_stacks; stack_number++)
        {
            stack_properties.at(stack_number).min_index = stack_number * SizeOfEachStack;
            stack_properties.at(stack_number).current_index = stack_properties.at(stack_number).min_index;
            stack_properties.at(stack_number).max_index = stack_properties.at(stack_number).min_index + SizeOfEachStack - 1;
        }
    };

    // Print function to print the internal arrays.
    void print_internal_arrays()
    {
        for(std::size_t stack_number = 0; stack_number < num_of_stacks; stack_number++)
        {
            std::print("Stack {}: {}n", stack_number, stack_properties.at(stack_number));
        }
    }

private:
    static constexpr size_t num_of_stacks{3};
    std::array<ThreeStack::StackProperties, num_of_stacks> stack_properties{};
};

And here is the std::formatter I created and which does not work:

// Create a std::formatter for the ThreeStack::StackProperties struct for any ThreeStack<ValueType, SizeOfEachStack>.
template <typename ValueType, std::size_t SizeOfEachStack>
struct std::formatter<ThreeStack<ValueType, SizeOfEachStack>::StackProperties>
{
    constexpr auto parse(format_parse_context& ctx) { return ctx.begin(); }

    template <typename FormatContext>
    auto format(const ThreeStack<ValueType, SizeOfEachStack>::StackProperties& stack_properties, FormatContext& ctx)
    {
        return format_to(ctx.out(), "min_index: {}, max_index: {}, current_index: {}", stack_properties.min_index, stack_properties.max_index, stack_properties.current_index);
    }
};

I got the below compile error for this std::formatter:

<source>:43:78: error: type/value mismatch at argument 1 in template parameter list for 'template<class _Tp, class _CharT> struct std::formatter'
   43 | struct std::formatter<ThreeStack<ValueType, SizeOfEachStack>::StackProperties>
      |                                                                              ^
<source>:43:78: note:   expected a type, got 'ThreeStack<ValueType, SizeOfEachStack>::StackProperties'
<source>:43:13: error: template class without a name
   43 | struct std::formatter<ThreeStack<ValueType, SizeOfEachStack>::StackProperties>
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Compiler returned: 1

https://godbolt.org/z/qeKsMrb4Y