scoping static_assert for class static member array

Question

I'd like to enforce a compile-time constraint about the length of a class static member variable array using static_assert(). However, the length of the variable named lpfilter[] is not known in the declaration in the header, and only known later in the definition. I'm uncertain how best to implement this static_assert so that it may see the static private array length. I have a workaround using a private scoped function to hold static_asserts, but is there a better way? I'm using clang 15 in C++14 mode.

//in header
class test
{ 
private:
    //member constants
    static const float lpfilter[];

    //feels like a workaround
    void testChecker(void);
};

//early in cpp file
const float test::lpfilter[] = {
    #include "a_single_line_lpfilter.csv"
};

I've made a few attempts so far. Each build error makes sense:

//after lpfilter[] definition in cpp file

//error: 'lpfilter' is a private member of 'test'
static_assert((sizeof(test::lpfilter)/sizeof(test::lpfilter[0])) % 2, "filter length must be even");

//error: redefinition of 'test'
class test
{
static_assert((sizeof(test::lpfilter)/sizeof(test::lpfilter[0])) % 2, "filter length must be even");
}

//error: expected unqualified-id
test::static_assert((sizeof(test::lpfilter)/sizeof(test::lpfilter[0])) % 2, "filter length must be even");

//works, but I'd prefer not to have to use a function to do this
void test::testChecker()
{
static_assert((sizeof(test::lpfilter)/sizeof(test::lpfilter[0])) % 2, "filter length must be even");
}