This should work:

@safe unittest
{
    import std.algorithm;
    assert(min(1.0, double.nan) == 1.0);
    assert(min(double.nan, 1.0) is double.nan);
    static struct A {
        int x;
        string y;
        int opCmp(A a) { return int(x > a.x) - int(x < a.x); }
    }
    assert(min(A(1, "first"), A(1, "second")) == A(1, "first"));
}

In other words, min should return its first argument unless it makes a positive determination that the second argument "deserves" to be returned instead.