`Map.+=` not a member of `Map[String, String]` (using scala2-library-tasty)

[nicolasstucki]

Compiler version

3.4.0-RC2

Minimized code

// Test.scala

def test: Unit =
  val sb = new StringBuilder
  var key = ""
  var map = Map.empty[String, String]
  map += key -> sb.toString()

sbt> set ThisBuild/Build.scala2Library := Build.Scala2LibraryCCTasty
sbt> scala3-bootstrapped/scalac Test.scala

Output

-- [E008] Not Found Error: Test.scala:5:6 ------------------------------------
5 |  map += key -> sb.toString()
  |  ^^^^^^
  |value += is not a member of Map[String, String] - did you mean Map[String, String].!=? or perhaps Map[String, String].++ or Map[String, String].++ or Map[String, String].==?

Expectation

Should compile as it does when we use the Scala 2 compiled library.

[nicolasstucki]

With the Scala 2 compiled library we get the transformation to map = map.+

map = map.+[String](ArrowAssoc[String](key).->[String](sb.toString()))

while with the TASTy library we fail to type map.+=

map.+=(ArrowAssoc[String](key).->[String](sb.toString))

[nicolasstucki]

The issue is that StringBuilder defines a toString without parenthesis and we use it with parenthesis.

The original code can be fixed with

-  map += key -> sb.toString()
+  map += key -> sb.toString

We need a better error message and we might need to revise which definitions in the library should have or not parenthesis.

[nicolasstucki]

Minimization

trait Map {
  def +(x: Any): Map
}
trait StringBuilder {
  def toString: String
}
def test(sb: StringBuilder): Unit =
  var map: Map = ???
  map += sb.toString()

[nicolasstucki]

Using

trait StringBuilder {
  def toString(): String
}

seems to make both map += sb.toString and map += sb.toString() work.