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

No warning because it is false. It's never too late to push back against false claims by tools and politicians.

The method is already known to be used under -Wunused. This warning could be under the same flag.

Note that the user might want an unusable extension because it is usable under an older JVM or other dependency. It might be nicer to make the warning "opt-in".

[som-snytt]

Just watched the sjrd talk on compatibility, linked from a lihaoyi sip.

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

I've been saying "MiMa" wrong all this time. It's pronounced like Jemima.

Some fun with captions, which I had on because of hearing decline:

"scalac people get it wrong sometimes"
"scholars see people get it wrong sometimes"

"JVM's point of view" is "the deviant's point of view".
"def" is "death".