isConstraint

@constraint static struct aConstraint
{
    bool check(int)
    {
        return false;
    }

    enum string errorMessage = "hi";
}

static struct notAConstraint
{
    bool check(int)
    {
        return false;
    }

    enum string errorMessage = "hi";
}

assert(isConstraint!(minLength));
assert(isConstraint!(aConstraint));
assert(!(isConstraint!(notAConstraint)));