Compiler bug for updateDynamic invocations within expressions

[scabug]

Problem with updateDynamic:
\
If a dynamic field assignment is invoked within an expression (the type of the assignment invocation depends on the result type of method updateDynamic), then the compiler reports the error reassignment to val.

scala> val foo = new A // A implements selectDynamic and updateDynamic
foo: A = A@71933d6c

scala> foo.xxx = 3
foo.xxx: Any = 3

scala> foo.xxx
res0: Any = 3

scala> println(foo.xxx = 3)
<console>:12: error: reassignment to val
              println(foo.xxx = 3)
                              ^

scala> val x = (foo.xxx = 3) == ()
<console>:11: error: reassignment to val
       val x = (foo.xxx = 3) == ()
                        ^

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-5733?orig=1
Reporter: @dgruntz
Affected Versions: 2.10.0-M3

[scabug]

@dgruntz said:
Actually, the latter assignments are not translated by the compiler to invocations of updateDynamic. If we look at the tree after typers, it looks as follows:

    private[this] val foo: A = new A();
    Test.this.foo.updateDynamic("xxx")(3);
    scala.this.Predef.println(foo.xxx = 3);
    private[this] val x: <error> = foo.xxx = 3.==()

[scabug]

@harrah said:
Can you provide an implementation of A that demonstrates this issue?

[scabug]

Alexis Agahi (alag) said (edited on Oct 14, 2012 1:04:21 PM UTC):
Can't figure out why updateDynamic works in App and wont compile the test class...

// src/AB.scala
import language.dynamics

object A extends Dynamic {
  var a = "a"

  def selectDynamic(method:String): String = a

  def updateDynamic(method:String)(v:String) { a = v }
}

class B extends Dynamic {
  var b = "b"

  def selectDynamic(method:String): String = b

  def updateDynamic(method:String)(v:String) { b = v }
}

object TestApp extends App {
  assert( A.foo == "a" )
  assert( A.bar == "a" )
  A.aaa = "aaa"
  assert( A.bar == "aaa" )


  val b = new B
  assert( b.foo == "b" )
  assert( b.bar == "b" )
  b.bbb = "bbb"
  assert( b.bar == "bbb" )

  println("Done")
}

// test/ABTest.scala
import org.junit.Test
import language.dynamics

class ABTest {

  @Test
  def test() {
    assert( A.foo == "a" )
    assert( A.bar == "a" )
    A.aaa = "aaa"
    assert( A.bar == "aaa" )


    val b = new B
    assert( b.foo == "b" )
    assert( b.bar == "b" )
    b.bbb = "bbb"
    assert( b.bar == "bbb" )

  }

}

// build.sbt
scalaVersion := "2.10.0-RC1"

scalaSource in Compile <<= baseDirectory(_ / "src")

scalaSource in Test <<= baseDirectory(_ / "test")

libraryDependencies += "com.novocode" % "junit-interface" % "0.10-M1" % "test"

libraryDependencies += "org.scala-lang" % "scala-reflect" % "2.10.0-RC1"

libraryDependencies += "org.scala-lang" % "scala-compiler" % "2.10.0-RC1"

> scala-version
[info] 2.10.0-RC1
> sbt-version
[info] 0.12.1
> test
[info] Compiling 1 Scala source to /2.10/target/scala-2.10/test-classes...
[error] /2.10/test/ABTest.scala:24: reassignment to val
[error]     A.aaa = "aaa"
[error]           ^
[error] /2.10/test/DynamicTest.scala:31: reassignment to val
[error]     b.bbb = "bbb"
[error]           ^
[error] two errors found
[error] (test:compile) Compilation failed

> clean
[success] Total time: 0 s, completed 14 oct. 2012 00:18:51
> run
[info] Updating {file:/2.10/}default-5d6b7a...
[info] Resolving org.scala-tools.testing#test-interface;0.5 ...
[info] Done updating.
[info] Compiling 1 Scala sources to /2.10/target/scala-2.10/classes...
[info] Running TestApp 
Done
[success] Total time: 17 s, completed 14 oct. 2012 00:19:09

[scabug]

@paulp said:
scala/scala#1681

[scabug]

@retronym said:
scala/scala@edbcc64