I wanted to reply that your point 3 should already be possible with C++ constexpr functions because it doesn't allow undefined behavior. But I it seems I was wrong about that or maybe I'm doing it wrong:

    [[nodiscard]] constexpr uint64_t
    getBits( uint8_t nBits )
    {
        return BITBUFFER >> ( 64 - nBits ) & ( ( 1ULL << nBits ) - 1U );
    }
 
    int main()
    {
        std::cerr << getBits( 0 ) << "\n";
        std::cerr << getBits( 1 ) << "\n";
        return 0;
    }

The first output will print a random number, 140728069214376 in my case, while the second line will always print 1. However, when I put the ( ( 1ULL << nBits ) - 1U ) part into a separate function and print the values for that, then getBits( 0 ) suddenly always returns 0 as if the compiler understands suddenly that it will and with 0.

    template<uint8_t nBits>
    [[nodiscard]] constexpr uint64_t
    getBits2()
    {
        return BITBUFFER >> ( 64 - nBits ) & ( ( 1ULL << nBits ) - 1U );
    }

In this case, the compiler will only print a warning when trying to call it with getBits2<0>. And here I kinda thought that constexpr would lead to errors on undefined behavior, partly because it always complains about uninitialized std::array local variables being an error. That seems inconsistent to me. Well, I guess that's what -Werror is for ...

Compiled with -std=c++17 and clang 16.0.0 on godbolt: https://godbolt.org/z/qxxWW93Tx