Non-final implicit classes in stdlib-bootstrapped (scala#17983)

Author: nicolasstucki

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

@@ -811,12 +811,13 @@ object desugar {
                     // TODO: drop this once we do not silently insert empty class parameters anymore
           case paramss => paramss
         }
+        val finalFlag = if ctx.settings.Yscala2Stdlib.value then EmptyFlags else Final
         // implicit wrapper is typechecked in same scope as constructor, so
         // we can reuse the constructor parameters; no derived params are needed.
         DefDef(
           className.toTermName, joinParams(constrTparams, defParamss),
           classTypeRef, creatorExpr)
-          .withMods(companionMods | mods.flags.toTermFlags & (GivenOrImplicit | Inline) | Final)
+          .withMods(companionMods | mods.flags.toTermFlags & (GivenOrImplicit | Inline) | finalFlag)
           .withSpan(cdef.span) :: Nil
       }
 

project/TastyMiMaFilters.scala

@@ -131,9 +131,6 @@ object TastyMiMaFilters {
     ProblemMatcher.make(ProblemKind.MissingTermMember, "scala.collection.convert.JavaCollectionWrappers.IterableWrapperTrait.iterator"),
     ProblemMatcher.make(ProblemKind.IncompatibleTypeChange, "scala.util.matching.Regex.MatchIterator.replacementData"), // before: scala.Any; after: scala.collection.AbstractIterator[scala.util.matching.Regex] & scala.util.matching.Regex.Replacement
 
-    // Problem: implicit class (method should not be final)
-    ProblemMatcher.make(ProblemKind.FinalMember, "scala.collection.convert.*.*"),
-
     // Problem: implicit class
     ProblemMatcher.make(ProblemKind.MissingTermMember, "scala.collection.generic.IsIterableLowPriority.is*LikeIsIterable"),
     ProblemMatcher.make(ProblemKind.MissingTermMember, "scala.collection.generic.IsIterableOnce.iterableOnceIsIterableOnce"),