False positive warning 'Extension method will never be selected'

[SimY4]

Compiler version

Observing this failure since Scala 3.4, the error persisted all the way till 3.6.1

Minimized code

trait A extends Iterable[String]

trait Test {
  extension (a: A) def foo: String = "foo"
}

object Test {
  def foo(s: String, acc: List[String] = Nil) = ???
}

Output

a false positive compiler warning:

[Warn] /.../src/main/scala/Test.scala:6:24: Extension method foo will never be selected

Expectation

The code works as expected after compilation. Extension method is always rightfully selected. The actual code that triggers it in my project is even weirder because it's nested deeper inside companion object like so:

trait A extends Iterable[String]

trait Test {
  extension (a: A) def foo: String = "foo"
}

object Test {
  object Nested {
    def foo(s: String, acc: List[String] = Nil) = ???
  }
}

still triggers the same error.

[som-snytt]

@SimY4 I'm unable to reproduce the symptom. How are you compiling it? Or if you can link to a sample project I'll try that.

[Gedochao]

@SimY4 I can't seem to be able to reproduce, either.
Are you passing any compiler flags?

[SimY4]

@som-snytt @Gedochao I'm having a hard time extracting it into a self contained project. I'll need to figure out what is it in my project that triggers it. I'll come back to you.

[som-snytt]

Glad to pursue with a test case.

[SimY4]

@som-snytt

So even smaller example is this one:

case class CC(a: String, b: String) extends Iterable[String] {
  override def iterator: Iterator[String] = Iterator(a, b)
}

trait T {
  extension (cc: CC) def className: String = "foo"
}

object O extends T {
  def foo = {
    val cc = CC("a", "b")
    cc.className
  }
}

O.foo

Look like it's reliably reproducible. The bit I missed is that the method name is important and the conflict occurs due to the fact that Iterable interface provides a method called def className: String So there's a conflict and a warning suggests that extension won't be preferred in scope of T but it is.

[Gedochao]

@SimY4 okay, I can reproduce with the new example.
Slight adjustment, as what you pasted is a script.
Here's a raw .scala minimization:

//> using scala 3.5.0
case class CC(a: String, b: String) extends Iterable[String] {
  override def iterator: Iterator[String] = Iterator(a, b)
}

trait T {
  extension (cc: CC) def className: String = "foo"
}

object O extends T {
  def foo = {
    val cc = CC("a", "b")
    println(cc.className)
  }
}

@main def main() = O.foo

And indeed, I'm getting the warning:

-- [E194] Potential Issue Warning: /Users/pchabelski/IdeaProjects/scala-cli-tests-2/untitled/compiler-repro/repro.scala:6:25 
7 |  extension (cc: CC) def className: String = "foo"
  |                         ^
  |Extension method className will never be selected
  |because CC already has a member with the same name and compatible parameter types.
  |-----------------------------------------------------------------------------
  | Explanation (enabled by `-explain`)
  |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
  | An extension method can be invoked as a regular method, but if that is intended,
  | it should not be defined as an extension.
  | Although extensions can be overloaded, they do not overload existing member methods.
   -----------------------------------------------------------------------------
1 warning found

Looks like a bug, indeed.

[Gedochao]

Observing this failure since Scala 3.4, the error persisted all the way till 3.6.1

So I dunno about 3.4.x, as I can't reproduce it before 3.5.0.
So it's potentially a regression.

Last good stable release: 3.4.3 (or chronologically, 3.4.2)
First bad stable release: 3.5.0
Last good nightly release: 3.5.0-RC1-bin-20240408-1e8a653-NIGHTLY
First bad nightly release: 3.5.0-RC1-bin-20240409-2148c8d-NIGHTLY

Bisect failed to pinpoint the exact commit, the culprit could be any of:

• 939f73e
• b006ef9
• fdb33a8
• 2148c8d

939f73e seems like it could be connected, so I'd guess it's the first to check.
cc @smarter

[som-snytt]

The PR was on 3.5, but I see it was backported to LTS 3.3.

(It feels longer ago because it began in May 2023 but not merged until April 2024.)

The check doesn't consider access.

The feature request added to the ticket was not to warn for simple forwarder, which might be nice to add:

class Foo(val someField: Int)

extension (foo: Foo)
  def someField = foo.someField
  def bar1 = 1 + someField
  def bar2 = 2 + someField
  def bar3 = 3 + someField

Arguably, it's also about access.