Since we have the pattern of returning float.nan from opCmp to signal incomparable values, it makes sense to have a trait to identify this behavior. Something like this:

template hasNanComparison(T1, T2 = T1)
{
    static if (isFloatingPoint!T1)
        enum hasNanComparison = true;
    else static if (!isAggregateType!T1)
        enum hasNanComparison = false;
    else static if (!is(typeof((T1 a, T2 b) => a.opCmp(b))))
        enum hasNanComparison = false;
    else
        enum hasNanComparison = hasNanComparison!(ReturnType!((T1 a, T2 b) => a.opCmp(b)));
}

///
@safe unittest
{
    assert( hasNanComparison!float);
    assert( hasNanComparison!real);
    assert(!hasNanComparison!int);
    assert(!hasNanComparison!string);
    assert(!hasNanComparison!(int*));
    assert(!hasNanComparison!(int[]));

    struct S
    {
        float opCmp(S s)
        {
            return float.nan;
        }
    }
    assert( hasNanComparison!S);

    struct S2
    {
        S opCmp(S2 s)
        {
            return S();
        }
    }
    assert( hasNanComparison!S2);
}