C++ Compile-Time Class With std::vector Member variable

I am trying to make a writer class, which would be used to turn data into a binary stream, at compile-time using a std::vector, and then converting it to std::array to be used at runtime. eg:

#include <cstdint>
#include <array>
#include <vector>

class Writter
{
   private:
    std::vector<uint8_t> vec;

   public:
    constexpr Writter() {}

    constexpr const std::vector<uint8_t> getVector() const { return (vec); }

    constexpr void write(uint8_t val) { vec.push_back(val); }

    constexpr size_t getVecSize() const { return (vec.size()); }
};

consteval Writter toConstant()
{
    Writter wr;

    //wr.write(1);

    return(wr);
}

consteval auto getBinary()
{
    constexpr Writter               wr = toConstant();
    constexpr std::vector<uint8_t>  vec = wr.getVector();
    constexpr size_t                vecSize = vec.size();
    std::array<uint8_t, vecSize>    arr;

    for(size_t i = 0; i < vecSize; ++i)
    {
        arr[i] = vec[i];
    }
    
    return arr;
}

int main()
{
    constexpr auto binary = getBinary();
}

The current version compiles, but as soon as you try to use the wr.write(1) member function, compilation fails with the given output:

/opt/compiler-explorer/gcc-trunk-20240725/include/c++/15.0.0/bits/allocator.h:193:52: error: 'toConstant()()' is not a constant expression because it refers to a result of 'operator new'
  193 |             return static_cast<_Tp*>(::operator new(__n));
      |                                      ~~~~~~~~~~~~~~^~~~~
<source>:39:12: error: invalid types 'int[size_t {aka long unsigned int}]' for array subscript
   39 |         arr[i] = vec[i];

I have tried to use different containers, or just having a raw pointer and managing the memory myself, but the problem always seems to be that the compiler doesnt understand that the std::vector member variable (or any other container), can be evaluated at compile time. Is there any way to make the compiler understand that it is within a constexpr context?