Get closest point in collider

I had this problem today, I'm also using 2018.3. Here's a helper function here that supplants collider2D.ClosestPoint. Edit the colliders as you need.

public static class Collider2DExtension
{
    /// <summary>
    /// Return the closest point on a Collider2D relative to point
    /// </summary>
    public static Vector2 ClosestPoint(this Collider2D col, Vector2 point)
    {
        GameObject go = new GameObject("tempCollider");
        go.transform.position = point;
        CircleCollider2D c = go.AddComponent<CircleCollider2D>();
        c.radius = 0.1f;
        ColliderDistance2D dist = col.Distance(c);
        Object.Destroy(go);
        return dist.pointA;
    }
}

sourced it from this thread: https://forum.unity.com/threads/get-closest-point-of-collider2d.225120/