Extension method name conflict produces Lambda toString with Java 17 bytecode instead of compile error

[carueda]

Compiler version

Scala 3.3.4 and 3.7.0 (likely other 3.x)

Minimized code

extension (s: String)
  def indent: String = s.replaceAll("\n", "\n  ")
  def preIndent: String = "  " + s.indent  // Calls extension method on same receiver

@main def test(): Unit =
  val result = "foo\nbar".preIndent
  println(result)

Output

Bug$package$$$Lambda$1/0x0000000800001800@17ed40e0

Using scala-cli:

Scala 3.3.4:

❯ scala-cli run Bug.scala --scala-version 3.3.4 -O -java-output-version:17
Compiling project (Scala 3.3.4, JVM (17))
Compiled project (Scala 3.3.4, JVM (17))
  Bug$package$$$Lambda$1/0x0000000800001800@17ed40e0

Scala 3.7.0:

❯ scala-cli run Bug.scala --scala-version 3.7.0 -O -java-output-version:17
Compiling project (Scala 3.7.0, JVM (17))
[warn] ./Bug.scala:2:7
[warn] Extension method indent will never be selected from type String
[warn] because String already has a member with the same name and compatible parameter types.
[warn]   def indent: String = s.replaceAll("\n", "\n  ")
[warn]       ^
Compiled project (Scala 3.7.0, JVM (17))
  Bug$package$$$Lambda$1/0x0000000800001800@17ed40e0

Expectation

foo
bar

When an extension method conflicts with an existing type member (e.g., String.indent), the compiler warns that it "will never be selected" but then generates broken code (Lambda toString) when another extension method tries to call it, instead of either making it work or failing with a compile error.

Workaround: Inline the logic instead of calling the other extension method:

  def preIndent: String = "  " + s.replaceAll("\n", "\n  ")

Note: Works correctly with -java-output-version:8:

❯ scala-cli run Bug.scala --scala-version 3.7.0 -O -java-output-version:8
Compiling project (Scala 3.7.0, JVM (17))
Compiled project (Scala 3.7.0, JVM (17))
  foo
  bar

[som-snytt]

You can see the result better with

println(s"[[$result]]")

It is just spaces plus "thing".indent.toString

// Calls extension method on same receiver

The comment is an expectation, not what it does.

indent is
public String indent(int n)

[som-snytt]

or failing with a compile error

Worth adding that -Werror is to boost the warning.

Also worth adding that a previous enhancement request was for -release to define the platform API supported by Scala.

[som-snytt]

Reopened for further discussion in case further action is desired.

In particular, the form without explicit selection is pernicious:

extension (s: String)
  def indent: String = s.replaceAll("x", "y")
  def m: String = "m" + s.indent
  def n: String = "n" + indent