`Seq.++` leaks private symbols to infered type signature

[WojciechMazur]

Based on failure in OpenCB for sbt/sbt - build logs

Compiler version

Last good release: 3.8.0-RC1-bin-20250818-aaa39c5-NIGHTLY
First bad release: 3.8.0-RC1-bin-20250819-1f13619-NIGHTLY
Bisect points to 1f13619

I have not inspected the ++ signature but probably it captures the argument. The infered reference to private symbol is probably not stripped.
It needs to be fixed before 3.8 - the opt-in experimental.cc is currently affecting the default compilation mode.

Minimized code

class Setting

object Test:
  private lazy val settings: Seq[Setting] = Nil
  
  def fails = Seq.empty[Setting] ++ settings
  def works: Seq[Setting] = Seq.empty[Setting] ++ settings

Output

Compiling project (Scala 3.8.0-RC1-bin-20250907-8c9c051-NIGHTLY, JVM (21))
[error] ./test.scala:6:7
[error] non-private method fails in object Test refers to private lazy value settings
[error] in its type signature => Seq[Setting]
[error]   def fails = Seq.empty[Setting] ++ settings
[error]       ^

Expectation

The typer should strip the captured symbol when CC is not enabled

[noti0na1]

The error can be bypassed if the private field is assigned to a local variable first.

When the result type refers to its parameters as singleton types, the private reference error can always happen.
We are not able to tell if a type is inferred or not in checkNoPrivateLeaks.

For a simple fix, can we skip all retain annotations for private leak check and drop inferred when CC is not enabled?

cc @odersky

[Gedochao]

cc @Linyxus @natsukagami