Don't warn on private proxy extension idiom

[som-snytt]

Compiler version

3.8

Minimized code

class Foo(val someField: Int)

extension (foo: Foo)
  def someField = foo.someField // warns even if private
  def bar1 = 1 + someField
  def bar2 = 2 + someField
  def bar3 = 3 + someField

#17543 (comment)

Output

5 |  private def someField = foo.someField
  |              ^
  |Extension method someField will never be selected from type Foo
  |because Foo already has a member with the same name and compatible parameter types.

Expectation

The someField extension is a trick to turn the references in barN to foo.someField, which as per the definition of the someField extension is the desired result.

Possibly, leveraging the trick should be permitted without warning if the extension is @unused or if the rhs is a forwarding expression.

[som-snytt]

Just watched the sjrd talk on compatibility.

ExtensionNullifiedByMember arrived 6 yrs after the compatibility talk (which highlights that problem).

"scalac people get it wrong sometimes"
"scholars see people get it wrong sometimes" (captioned)

[som-snytt]

The example on the other ticket where the shadowing is unintended.

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

Perhaps there should be no warning if the extension is marked @unused or its rhs is a simple forwarder.