Can't read a constant value in compile time?

[Adnan via Digitalmars-d-learn]

Godbolt: https://godbolt.org/z/SWWOu7
When I write `something!(aNumber)()` and if a number is an 
immutable/enum it should be able to be read at compile time, 
right? Why is this different?

auto fizzbuzz(uint N)() {
    static string accumulate;
    return fizzbuzz!N(accumulate);
}

auto fizzbuzz(uint N)(ref string result) if (N % 3 && N % 5) {
    import std.conv : to;

    result ~= N.to!string ~ "\n";
    return fizzbuzz!(N - 1)(result);
}

auto fizzbuzz(uint N)(ref string result) if (!(N % 15)) {
    result ~= "FizzBuzz\n";
    return fizzbuzz!(N - 1)(result);
}

auto fizzbuzz(uint N)(ref string result) if (!(N % 3) && N % 5) {
    result ~= "Fizz\n";
    return fizzbuzz!(N - 1)(result);
}

auto fizzbuzz(uint N)(ref string result) if (!(N % 5) && N % 3) {
    result ~= "Buzz\n";
    return fizzbuzz!(N - 1)(result);
}

auto fizzbuzz(uint N : 0)(ref string result) {
    return result;
}

void main() {
    import std.stdio : writeln;

    enum lmao = fizzbuzz!50();

    lmao.writeln();
}