I have the following function:

constexpr int count_format_placeholders(const std::string_view format)
{
    int count = 0;
    for(int i = 0; i < format.size(); ++i)
    {
        if(format[i] == '{' && format[i + 1] == '}')
        {
            ++count;
            ++i;
        }
    }
    return count;
}

I have defined two functions, func and func2:

template<typename... Args>
constexpr int func(std::string_view format)
{
    return count_format_placeholders(format);
}

template<typename... Args>
constexpr int func2(std::string_view format)
{
    constexpr int i = count_format_placeholders(format);
    return i;
}

int main()
{
    constexpr std::string_view format{"{} {} {} {}:{}:{}"};

    constexpr int num_placeholders = count_format_placeholders(format);
    std::cout << num_placeholders << std::endl;
    constexpr int num_placeholders2 = func<int, int>(format);
    std::cout << num_placeholders2 << std::endl;
    constexpr int num_placeholders3 = func2<int, int>(format);
    std::cout << num_placeholders3 << std::endl;
    return 0;
}

The direct call to count_format_placeholders, and func work fine. Calling func2, however, generates a format is not constant expression error.

Why is this, and how can I fix this?

Easy testing: https://godbolt.org/z/9zoWM95hh