MessagePack Fallback decoder uses path label "either" in error, and the size error says "1 or 2" instead of "2 or 3"

[haskiindahouse]

Two small but confusing copy-paste leftovers in the MsgPack decodeFallback:

1. When an unexpected field name is encountered inside a Fallback structure, the error path appends "either" instead of "fallback". Looks like a copy from decodeEither right above it.
2. When the array header size is unrecognized, the error message says "Expected 1 or 2 elements but received $size", but the code actually handles size 2 (Left/Right) and size 3 (Both). The "1 or 2" wording is misleading.

Source

zio-schema/zio-schema-msg-pack/src/main/scala/zio/schema/codec/MessagePackDecoder.scala

Lines 327 to 349 in f32240d

	private def decodeFallback[A, B](path: Path, left: Schema[A], right: Schema[B], fullDecode: Boolean): Result[Fallback[A, B]] =
	Try(unpacker.unpackArrayHeader()).fold(
	err => fail(path, s"Error parsing Fallback structure: ${err.getMessage}"),
	size =>
	if (size == 2) {
	decodeString(path :+ "fallback").flatMap {
	case "left" => decodeValue(path :+ "fallback:left", left).map(Fallback.Left(_))
	case "right" => decodeValue(path :+ "fallback:right", right).map(Fallback.Right(_))
	case str => fail(path :+ "either", s"Unexpected field name: $str")
	}
	} else if (size == 3) {
	unpacker.skipValue()
	if (fullDecode)
	decodeValue(path :+ "fallback:left", left).flatMap(l => decodeValue(path :+ "fallback:right", right).map(r => Fallback.Both(l, r)))
	else {
	val res = decodeValue(path :+ "fallback:left", left).map(Fallback.Left(_))
	unpacker.skipValue()
	res
	}
	} else {
	fail(path :+ "fallback", s"Expected 1 or 2 elements but received $size.")
	}
	)

private def decodeFallback[A, B](path: Path, left: Schema[A], right: Schema[B], fullDecode: Boolean): Result[Fallback[A, B]] =
  Try(unpacker.unpackArrayHeader()).fold(
    err => fail(path, s"Error parsing Fallback structure: ${err.getMessage}"),
    size =>
      if (size == 2) {
        decodeString(path :+ "fallback").flatMap {
          case "left"  => decodeValue(path :+ "fallback:left", left).map(Fallback.Left(_))
          case "right" => decodeValue(path :+ "fallback:right", right).map(Fallback.Right(_))
          case str     => fail(path :+ "either", s"Unexpected field name: $str")  // <-- "either" should be "fallback"
        }
      } else if (size == 3) {
        // ...
      } else {
        fail(path :+ "fallback", s"Expected 1 or 2 elements but received $size.")  // <-- should be "2 or 3"
      }
  )

Expected

• The path on the unexpected-field-name failure says fallback, matching the rest of the method.
• The size-mismatch message reads "Expected 2 or 3 elements but received $size.".

Actual

Misleading "either" path on field-name failures, misleading "1 or 2" count in the size-error.

Happy to PR.

Versions

zio-schema-msg-pack at the SHA above (current main), Scala 3.8.3.