String Array Concatenation result modified by Static Compilation

import groovy.transform.CompileStatic

@CompileStatic
public class Concat {
  public static void main(String[] args) {
    def x = 'a,b,c'.split(',')
    def y = 'd,e,f'.split(',')
    println(x + y)
  }
} 

[a, b, c, d, e, f]

[a, b, c, [d, e, f]]