CheckUnused reports retained bodies

[som-snytt]

Compiler version

3.7

Minimized code

//> using options -Werror -Wunused:privates

case class BinaryFen(value: Array[Byte]) extends AnyVal:

  def read: Unit =
    val reader = new Iterator[Byte]:
      val inner                            = value.iterator
      override inline def hasNext: Boolean = inner.hasNext // nowarn
      override inline def next: Byte       = if hasNext then inner.next else 0.toByte // nowarn

    if reader.hasNext then
      val b: Byte = reader.next
      println(b)

Output

-- [E198] Unused Symbol Warning: tests/warn/unused-inline-private.scala:9:45 ---
9 |      override inline def hasNext: Boolean = inner.hasNext // nowarn
  |                                             ^
  |                                             unused private member
-- [E198] Unused Symbol Warning: tests/warn/unused-inline-private.scala:10:45 --
10 |      override inline def next: Byte       = if hasNext then inner.next else 0.toByte // nowarn
   |                                             ^
   |                                             unused private member
2 warnings found

Expectation

Quick println shows that it is reporting

PRIVATE method next$retainedBody
PRIVATE method hasNext$retainedBody

but it should exclude those special methods.

Worth adding that the lichess code probably does not do what was probably expected.

  public final void read$extension(byte[]);
    descriptor: ([B)V
    flags: (0x0011) ACC_PUBLIC, ACC_FINAL
    Code:
      stack=3, locals=4, args_size=2
         0: new           #9                  // class BinaryFen$$anon$1
         3: dup
         4: aload_1
         5: invokespecial #98                 // Method BinaryFen$$anon$1."<init>":([B)V
         8: astore_2
         9: aload_2
        10: invokeinterface #104,  1          // InterfaceMethod scala/collection/Iterator.hasNext:()Z
        15: ifeq          39
        18: aload_2
        19: invokeinterface #107,  1          // InterfaceMethod scala/collection/Iterator.next:()Ljava/lang/Object;
        24: invokestatic  #111                // Method scala/runtime/BoxesRunTime.unboxToByte:(Ljava/lang/Object;)B
        27: istore_3
        28: getstatic     #116                // Field scala/Predef$.MODULE$:Lscala/Predef$;
        31: iload_3
        32: invokestatic  #120                // Method scala/runtime/BoxesRunTime.boxToByte:(B)Ljava/lang/Byte;
        35: invokevirtual #124                // Method scala/Predef$.println:(Ljava/lang/Object;)V
        38: return
        39: return

It uses the unspecialized next, which boxes the result of specialized next, which unboxes the result of underlying inner.next.

Also normally the anon iterator should extend AbstractIterator.