Imported From: https://issues.scala-lang.org/browse/SI-10077?orig=1
Reporter: Sergey Alaev (salaev)
Affected Versions: 2.12.0
See #9834
Attachments:

• example.tar.gz (created on Nov 25, 2016 7:42:47 PM UTC, 294 bytes)

@som-snytt said:
This is because x ^ y is an Int, not a Byte.

scala> arr1(4) = (arr1(4) ^ arr2(5)).toByte

scala> arr1.update(4, (arr1(4) ^ arr2(5)).toByte)

scala> implicit class cv(val as: Array[Byte]) extends AnyVal {
     | def update(i: Int, v: Int): Unit = as.update(i, v.toByte)
     | }
defined class cv

scala> arr1(4) ^= arr2(5)

Maybe it's possible to spec it in such a way that the conversions are inserted.

Anyway, the proposed fix for #9834 is slightly obscurantistical:

scala> arr1(4) ^= arr2(5)
<console>:14: error: value ^= is not a member of Byte
  Expression does not convert to assignment because:
    type mismatch;
     found   : Int
     required: Byte
       arr1(4) ^= arr2(5)
               ^

@SethTisue said:
You don't need Array to demonstrate the issue here:

scala> var b1 = 0.toByte; b1 ^= b1
<console>:13: error: type mismatch;
 found   : Int
 required: Byte
        b1 ^= b1
           ^

equivalent Java code compiles and the bytecode includes the necessary i2b instruction.

@SethTisue said:
I'm ambivalent about whether it's desirable for Scala to further paper over the lack of support for bytewise operations at the bytecode (ironic! "byte"code! hahaha!) level, especially since it would require additional language in the spec. (JLS 15.26.2 has "the result of the binary operation is converted to the type of the left-hand variable")

@som-snytt said:
The error message improvement now shows the expansion, which can be tricky to visualize.

scala> var b1 = 0.toByte ; b1 ^= b1
<console>:13: error: value ^= is not a member of Byte
  Expression does not convert to assignment because:
    type mismatch;
     found   : Int
     required: Byte
    expansion: this.b1 = this.b1.^(b1)
        b1 ^= b1
           ^

scala> val as, bs = new Array[Byte](10)
as: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
bs: Array[Byte] = Array(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

scala> as(4) ^= bs(5)
<console>:14: error: value ^= is not a member of Byte
  Expression does not convert to assignment because:
    type mismatch;
     found   : Int
     required: Byte
    expansion: as.update(4, as.apply(4).^(bs(5)))
       as(4) ^= bs(5)
             ^

It would be nice to highlight the error in the expansion without adding a caret line.

Closing as duplicate of the linked ticket by which the error message was improved.