Fix ArrayStoreException in ObjectArrays.concat(element, array) / concat(array, element)

guava-tests/test/com/google/common/collect/ObjectArraysTest.java

@@ -194,6 +194,30 @@ public void testAppendTwoElements() {
     assertThat(array).asList().containsExactly("foo", "bar", "baz").inOrder();
   }
 
+  public void testPrependZeroElements_withIncompatibleElementType() {
+    String[] array = {};
+    Object[] result = ObjectArrays.<Object>concat(0, array);
+    assertThat(result).asList().containsExactly(0);
+  }
+
+  public void testPrependOneElement_withIncompatibleElementType() {
+    String[] array = {"bar"};
+    Object[] result = ObjectArrays.<Object>concat(0, array);
+    assertThat(result).asList().containsExactly(0, "bar").inOrder();
+  }
+
+  public void testAppendZeroElements_withIncompatibleElementType() {
+    String[] array = {};
+    Object[] result = ObjectArrays.<Object>concat(array, 0);
+    assertThat(result).asList().containsExactly(0);
+  }
+
+  public void testAppendOneElement_withIncompatibleElementType() {
+    String[] array = {"foo"};
+    Object[] result = ObjectArrays.<Object>concat(array, 0);
+    assertThat(result).asList().containsExactly("foo", 0).inOrder();
+  }
+
   public void testEmptyArrayToEmpty() {
     doTestNewArrayEquals(new Object[0], 0);
   }

guava/src/com/google/common/collect/ObjectArrays.java

@@ -93,10 +93,18 @@ private ObjectArrays() {}
    *     the first position, and the elements of {@code array} occupying the remaining elements.
    */
   public static <T extends @Nullable Object> T[] concat(@ParametricNullness T element, T[] array) {
-    T[] result = newArray(array, array.length + 1);
+    Object[] result;
+    if (element == null || array.getClass().getComponentType().isInstance(element)) {
+      result = newArray(array, array.length + 1);
+    } else {
+      result = new Object[array.length + 1];
+    }
     result[0] = element;
     arraycopy(array, 0, result, 1, array.length);
-    return result;
+    @SuppressWarnings(
+        "unchecked") // safe: result's component type accepts `element` and all of `array`
+    T[] typedResult = (T[]) result;
+    return typedResult;
   }
 
   /**
@@ -108,9 +116,18 @@ private ObjectArrays() {}
    *     array}, plus {@code element} occupying the last position.
    */
   public static <T extends @Nullable Object> T[] concat(T[] array, @ParametricNullness T element) {
-    T[] result = Arrays.copyOf(array, array.length + 1);
+    Object[] result;
+    if (element == null || array.getClass().getComponentType().isInstance(element)) {
+      result = Arrays.copyOf(array, array.length + 1);
+    } else {
+      result = new Object[array.length + 1];
+      arraycopy(array, 0, result, 0, array.length);
+    }
     result[array.length] = element;
-    return result;
+    @SuppressWarnings(
+        "unchecked") // safe: result's component type accepts `element` and all of `array`
+    T[] typedResult = (T[]) result;
+    return typedResult;
   }
 
   /**