Why does array[idx++]+=“a” increase idx once in Ja

For a challenge, a fellow code golfer wrote the following code:

import java.util.*;
public class Main {
  public static void main(String[] args) {
    int size = 3;
    String[] array = new String[size];
    Arrays.fill(array, "");
    for(int i = 0; i <= 100; ) {
      array[i++%size] += i + " ";
    }
    for(String element: array) {
      System.out.println(element);
    }
  }
}

When running this code in Java 8, we get the following result:

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76 79 82 85 88 91 94 97 100 
2 5 8 11 14 17 20 23 26 29 32 35 38 41 44 47 50 53 56 59 62 65 68 71 74 77 80 83 86 89 92 95 98 101 
3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99

When running this code in Java 10, we get the following result:

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100 102 
2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88 90 92 94 96 98 100

The numbering is entirely off using Java 10. So what is happening here? Is it a bug in Java 10?

Follow ups from the comments:

The issue appears when compiled with Java 9 or later (we found it in Java 10). Compiling this code on Java 8, then running in Java 9 or any later version, including Java 11 early access, gives the expected result.
This kind of code is non-standard, but is valid according to the spec. It was found by Kevin Cruijssen in a discussion in a golfing challenge, hence the weird use case encountered.

Didier L found out that the issue can be reproduced with the much smaller and more understandable code:

class Main {
  public static void main(String[] args) {
    String[] array = { "" };
    array[test()] += "a";
  }
  static int test() {
    System.out.println("evaluated");
    return 0;
  }
}

Result when compiled in Java 8:

evaluated

Result when compiled in Java 9 and 10:

evaluated
evaluated

The issue seems to be limited to the string concatenation and assignment operator (+=) with an expression with side effect(s) as the left operand, like in array[test()]+="a", array[ix++]+="a", test()[index]+="a", or test().field+="a". To enable string concatenation, at least one of the sides must have type String. Trying to reproduce this on other types or constructs failed.

This is a bug in javac starting from JDK 9 (which made some changes with regard to string concatenation, which I suspect is part of the problem), as confirmed by the javac team under the bug id JDK-8204322. If you look at the corresponding bytecode for the line:

array[i++%size] += i + " ";

It is:

  21: aload_2
  22: iload_3
  23: iinc          3, 1
  26: iload_1
  27: irem
  28: aload_2
  29: iload_3
  30: iinc          3, 1
  33: iload_1
  34: irem
  35: aaload
  36: iload_3
  37: invokedynamic #5,  0 // makeConcatWithConstants:(Ljava/lang/String;I)Ljava/lang/String;
  42: aastore

Where the last aaload is the actual load from the array. However, the part

  21: aload_2             // load the array reference
  22: iload_3             // load 'i'
  23: iinc          3, 1  // increment 'i' (doesn't affect the loaded value)
  26: iload_1             // load 'size'
  27: irem                // compute the remainder

Which roughly corresponds to the expression array[i++%size] (minus the actual load and store), is in there twice. This is incorrect, as the spec says in jls-15.26.2:

A compound assignment expression of the form E1 op= E2 is equivalent to E1 = (T) ((E1) op (E2)), where T is the type of E1, except that E1 is evaluated only once.

So, for the expression array[i++%size] += i + " ";, the part array[i++%size] should only be evaluated once. But it is evaluated twice (once for the load, and once for the store).

So yes, this is a bug.