Diff on arrays with single values is reducing them to the value

[mcnerneyd]

json4s version

4.1.0-M3
4.0.6

scala version

2.13.8

jdk version

Open JDK 11

When I do a diff on two objects both containing the same key pointing to an array, and the new value contains only one value then the resulting change is that single value as opposed to an array containing that value.

The following test demonstrates the problem:

    val v0 = JsonMethods.parse("""{ "a": [8,9] }""")
    val v1 = JsonMethods.parse("""{ "a": [10,11] }""")

    val Diff(c1,a1,d1) = v0 diff v1
    println("     c:" + c1)
    println("     a:" + a1)
    println("     d:" + d1)
    c1 \ "a" shouldBe JArray(List(10, 11))

    val v2 = JsonMethods.parse("""{ "a": [10] }""")

    val Diff(c2,a2,d2) = v0 diff v2
    println("     c:" + c2)
    println("     a:" + a2)
    println("     d:" + d2)
    c2 \ "a"  shouldBe JArray(List(10))  // Fails
  }

The final value, rather than being [10] is just 10.

The problem appears to be on line 102 of Diff.scala (4.1.0-M3):

      case (xs, Nil) => Diff(JNothing, JNothing, if (xs.isEmpty) JNothing else JArray(xs))
      case (Nil, ys) => Diff(JNothing, if (ys.isEmpty) JNothing else JArray(ys), JNothing)
      case (x :: xs, y :: ys) =>
        val Diff(c1, a1, d1) = diff(x, y)
        val Diff(c2, a2, d2) = diffRec(xs, ys)
        Diff(c1 ++ c2, a1 ++ a2, d1 ++ d2)           // Line 102
    }

Where c1 is any value and c2 is nothing, the result is c1 when it should be a JArray of c1.

Possibly, a combiner method is needed that combines JValue with a JArray as normal, but a JValue with a JNothing as a new JArray.