Recover scala 2's semantics for AnyVal under -Ycompile-scala2-library (#22536)

Closes #22533

Author: hamzaremmal

compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala

@@ -78,7 +78,13 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
 
   private def existingDef(sym: Symbol, clazz: ClassSymbol)(using Context): Symbol =
     val existing = sym.matchingMember(clazz.thisType)
-    if existing != sym && !existing.is(Deferred) then existing else NoSymbol
+    if ctx.settings.YcompileScala2Library.value && clazz.isValueClass && (sym == defn.Any_equals || sym == defn.Any_hashCode) then
+      NoSymbol
+    else if existing != sym && !existing.is(Deferred) then 
+      existing 
+    else 
+      NoSymbol
+  end existingDef
 
   private def synthesizeDef(sym: TermSymbol, rhsFn: List[List[Tree]] => Context ?=> Tree)(using Context): Tree =
     DefDef(sym, rhsFn(_)(using ctx.withOwner(sym))).withSpan(ctx.owner.span.focus)

compiler/test/dotty/tools/dotc/printing/PrintingTest.scala

@@ -69,6 +69,9 @@ class PrintingTest {
   @Test
   def printing: Unit = testIn("tests/printing", "typer")
 
+  @Test
+  def posttyper: Unit = testIn("tests/printing/posttyper", "posttyper")
+
   @Test
   def untypedPrinting: Unit = testIn("tests/printing/untyped", "parser")
 

project/Scala2LibraryBootstrappedMiMaFilters.scala

@@ -15,8 +15,21 @@ object Scala2LibraryBootstrappedMiMaFilters {
         ProblemFilters.exclude[FinalClassProblem]("scala.language$experimental$"),
         ProblemFilters.exclude[FinalClassProblem]("scala.languageFeature$*$"),
 
-        // Value class extension methods
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*$extension"),
+        // Issue: https://github.com/scala/scala3/issues/22495
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.ArrayOps.scala$collection$ArrayOps$$elemTag$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.ArrayOps.iterateUntilEmpty$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.isLineBreak$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.isLineBreak2$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.linesSeparated$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.escape$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.toBooleanImpl$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.unwrapArg$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.StringOps.iterateUntilEmpty$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple2Zipped.coll1$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple2Zipped.coll2$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple3Zipped.coll1$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple3Zipped.coll2$extension"),
+        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.runtime.Tuple3Zipped.coll3$extension"),
 
         // Companion module class
         ProblemFilters.exclude[FinalClassProblem]("scala.*$"),

tests/printing/posttyper/i22533.check

@@ -0,0 +1,25 @@
+[[syntax trees at end of                 posttyper]] // tests/printing/posttyper/i22533.scala
+package <empty> {
+  @SourceFile("tests/printing/posttyper/i22533.scala") trait A() extends Any {
+    override def equals(x: Any): Boolean = ???
+    override def hashCode(): Int = ???
+  }
+  @SourceFile("tests/printing/posttyper/i22533.scala") final class Foo(u: Int)
+     extends AnyVal(), A {
+    override def hashCode(): Int = Foo.this.u.hashCode()
+    override def equals(x$0: Any): Boolean =
+      x$0 match
+        {
+          case x$0 @ _:Foo @unchecked => this.u.==(x$0.u)
+          case _ => false
+        }
+    private[this] val u: Int
+  }
+  final lazy module val Foo: Foo = new Foo()
+  @SourceFile("tests/printing/posttyper/i22533.scala") final module class Foo()
+     extends AnyRef() { this: Foo.type =>
+    private def writeReplace(): AnyRef =
+      new scala.runtime.ModuleSerializationProxy(classOf[Foo.type])
+  }
+}
+

tests/printing/posttyper/i22533.flags

@@ -0,0 +1 @@
+-Ycompile-scala2-library

tests/printing/posttyper/i22533.scala

@@ -0,0 +1,7 @@
+//> using options -Ycompile-scala2-library
+
+trait A extends Any:
+    override def equals(x: Any): Boolean = ???
+    override def hashCode(): Int = ???
+
+class Foo(u: Int) extends AnyVal, A