Inconsistent extension methods with same signature [on different types]

[antognini]

Everything compiled with 3.0.0-M3:

This fails to compile:

package ext

final case class A()
final case class B(a:A)

extension(a:A) 
  def x = 5

extension(b:B)
  def x = b.a.x
//        ^^^^^
//        value x is not a member of ext.A.
//        An extension method was tried, but could not be fully constructed:
//
//        b.a

However, with explicit return types [Int], it compiles:

package ext

final case class A()
final case class B(a:A)

extension(a:A)
  def x:Int = 5

extension(b:B)
  def x:Int = b.a.x

Furthermore, when the very same example is split into separate compilation units, it fails with yet another error message:
ExtensionA.scala

package ext

final case class A()

extension(a:A)
  def x:Int = 5

ExtensionB.scala

package ext
import ext.A

final case class B(a:A)

extension(b:B)
  def x:Int = b.a.x
//^^^^^^^^^^^^^^^^^
//x is already defined as method x in ext.ExtensionA.scala

Expectation

Leveraging same extension signatures [on different types] should be consistently defined (is it possible or not?)
The behaviour should not depend on using/not using type inference, nor on same/separate compilation units.

[antognini]

In other words, I expect the three given examples to either compile/work or fail with the same error message (or similar error messages).