Extension method name collision across different source files

[soronpo]

Different extension methods in the same package, with the same name, but in different files collide.
If defined in the same source file no error.

Compiler version

v3.0.1

Minimized code

Minimized project at: https://github.com/soronpo/dottybug/tree/extension_collision

Test/Foo.scala

package Test

trait Foo
extension (foo : Foo)
  def fails : Unit = {}
  def works : Unit = {}

extension (bar : Bar)
  def works : Unit = {}

Test/Bar.scala

package Test

trait Bar
extension (bar : Bar)
  def fails : Unit = {}

Output

[error] -- [E161] Naming Error: C:\IdeaProjects\dottybug\src\main\scala\Test\Foo.scala:5:6 ----------------------------------
[error] 5 |  def fails : Unit = {}
[error]   |  ^^^^^^^^^^^^^^^^^^^^^
[error]   |  fails is already defined as method fails in C:\IdeaProjects\dottybug\src\main\scala\Test\Bar.scala

Expectation

No error.

[odersky]

It is an error. See clause 4 in https://dotty.epfl.ch/docs/reference/dropped-features/package-objects.html

If several top-level definitions are overloaded variants with the same name, they must all come from the same source file.

I have changed the error message to add that as an additional note.

[soronpo]

What is the reason for this limitation?

[odersky]

Name resolution needs a single prefix to represent an overloaded reference. That prefix is the reference to the package object in which all alternatives are defined.