TABLE.COLUMN reference doesn't work with new columns created by UDF

// code placeholder
df1 = sc.parallelize(l).toDF("x","y").as("cat")
val squared = udf((s: Int) => s * s)
val df2 = df1.withColumn("z", squared(col("y")))
df2.columns //Array[String] = Array(x, y, z)

df2.select("cat.x") // works

df2.select("cat.z") // Doesn't work
// org.apache.spark.sql.AnalysisException: cannot resolve '`cat.z`' given input 
// columns: [cat.x, cat.y, z];;