Inline _N case class product members under -Yscala2-stdlib

Scala 2 does not generate the methods `_N`. This is solved by inlining
those calls, which removes the calls to those methods and the definitions
of those methods in the bytecode.

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -637,7 +637,10 @@ object desugar {
     //       new C[...](p1, ..., pN)(moreParams)
     val (caseClassMeths, enumScaffolding) = {
       def syntheticProperty(name: TermName, tpt: Tree, rhs: Tree) =
-        DefDef(name, Nil, tpt, rhs).withMods(synthetic)
+        val mods =
+          if ctx.settings.Yscala2Stdlib.value then synthetic | Inline
+          else synthetic
+        DefDef(name, Nil, tpt, rhs).withMods(mods)
 
       def productElemMeths =
         val caseParams = derivedVparamss.head.toArray

project/MiMaFilters.scala

@@ -185,12 +185,6 @@ object MiMaFilters {
         // Companion module class: Missing type java.io.Serializable
         ProblemFilters.exclude[MissingTypesProblem]("scala.*$"),
 
-        // Case class product accessors
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*._1"),
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*._2"),
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*._3"),
-        ProblemFilters.exclude[DirectMissingMethodProblem]("scala.*._4"),
-
         // abstract method elemTag()scala.reflect.ClassTag in class scala.collection.mutable.ArraySeq does not have a correspondent in other version
         ProblemFilters.exclude[DirectAbstractMethodProblem]("scala.collection.immutable.ArraySeq.elemTag"),
         ProblemFilters.exclude[DirectAbstractMethodProblem]("scala.collection.mutable.ArraySeq.elemTag"),

stdlib-bootstrapped/test/Main.scala

@@ -15,6 +15,7 @@ object HelloWorld:
 
     testScala2UnapplySignatures()
     testScala2ObjectParents()
+    testScala2ProductMembers()
   }
 
   def testScala2UnapplySignatures() = {
@@ -31,3 +32,11 @@ object HelloWorld:
     assert(!typeChecks("Either: scala.deriving.Mirror.Sum"))
     assert(!typeChecks("Either: scala.deriving.Mirror"))
   }
+  def testScala2ProductMembers() = {
+    Some(1)._1
+    Right(1)._1
+    (1, 2)._1
+    (1, 2)._2
+    ::(1, Nil)._1
+    ::(1, Nil)._2
+  }