Inlining failure when accessing same-named private members in sub and superclass

[WojciechMazur]

Based on the community build failure https://scala3.westeurope.cloudapp.azure.com/blue/organizations/jenkins/buildCommunityProject/detail/buildCommunityProject/1136/pipeline/
Library maintainer @tabdulradi

Problem exists only if both open classes define extension methods using type matching

Compiler version

Works in 3.1.3
Fails in 3.2.0-RC1 and RC2
Fails in 3.2.1-RC1-bin-20220706-9d07d52-NIGHTLY

Minimized code

import scala.reflect.TypeTest

open class ForSyntax[E](using E: TypeTest[E | Any, E]):
  extension [A](aOrE: E | A)
    inline def flatMap[B](f: A => E | B): E | B =
      aOrE match
        case e: E => ???
        case _ => ???


open class UnhappyCase[E](using E: TypeTest[E | Any, E]) extends ForSyntax[E]: 
  extension [A](aOrE: E | A) 
    inline def fold[B](inline fe: E => B, inline fa: A => B): B =
      aOrE match
        case e: E => ??? 
        case _ => ???

Output

[error] ./test.scala:11:33: error overriding method inline$E in class ForSyntax of type => reflect.TypeTest[E | Any, E];
[error]   method inline$E of type => reflect.TypeTest[E | Any, E] cannot override final member method inline$E in class ForSyntax
[error] open class UnhappyCase[E](using E: TypeTest[E | Any, E]) extends ForSyntax[E]: 
[error]                                 ^
Error compiling project (Scala 3.2.1-RC1-bin-20220706-9d07d52-NIGHTLY, JVM)

Expectation

Should compile

[KacperFKorban]

Minimization

A bit smaller example:

class Parent[E](using u: Unit):
  inline def m: Any = u

class Klass[E](using u: Unit) extends Parent[E]:
  inline def n: Any = u

Error:

-- [E164] Declaration Error: tests/pos/i15612.scala:4:21 -----------------------
4 |class Klass[E](using u: Unit) extends Parent[E]:
  |                     ^
  |error overriding method inline$u in class Parent of type => Unit;
  |  method inline$u of type => Unit cannot override final member method inline$u in class Parent

Explanation

The first commit that breaks this is: 13df1bc

The problem here is that the given vals are private and so the inline accessors are generated to be final not to override them by mistake.

Generated tree (after refchecks):

package <empty> {
  @SourceFile("tests/pos/i15612.scala") class Parent[E](using u: Unit) extends 
    Object
  () {
    E
    private[this] given val u: Unit
    inline def m: Any = this.inline$u:Any
    final def inline$u: Unit = Parent.this.u
  }
  @SourceFile("tests/pos/i15612.scala") class Klass[E](using u: Unit) extends 
    Parent[E]
  (u)() {
    E
    private[this] given val u: Unit
    inline def n: Any = this.inline$u:Any
    final def inline$u: Unit = Klass.this.u
  }
}

Workaround

A possible workaround here is to make those givens protected and override them explicitly. (since semantically those TypeTests are the same for us)
Like so:

class Parent[E](using protected val u: Unit):
  inline def m: Any = u

class Klass[E](using override protected val u: Unit) extends Parent[E]:
  inline def n: Any = u

[odersky]

Minimization:

class A:
  private val a = 1
  inline def foo() = a
class B extends A:
  private val a = 2
  inline def bar() = a