countsAtMost

static void test(R)(R x)
    if (isInputRange!R)
{
    import std.algorithm.searching : count;
    immutable n = x.count;

    // below
    foreach (immutable i; 0 .. n)
    {
        assert(x.countsAtLeast(i));
        assert(!x.countsExactly(i));
        assert(!x.countsAtMost(i));
    }

    // at
    assert(x.countsAtLeast(n));
    assert(x.countsExactly(n));
    assert(x.countsAtMost(n));

    // above
    foreach (immutable i; n + 1 .. n + 10)
    {
        assert(!x.countsAtLeast(i));
        assert(!x.countsExactly(i));
        assert(x.countsAtMost(i));
    }
}

import std.algorithm.iteration : filter;
import std.range : iota;

test(3.iota.filter!(x => true));

int[3] x = [0, 1, 2];
test(x[]);