partially address review comments

Author: olhotak

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -663,7 +663,8 @@ class TypeErasure(sourceLanguage: SourceLanguage, semiEraseVCs: Boolean, isConst
       case tp: TypeVar if !tp.isInstantiated =>
         assert(inSigName, i"Cannot erase uninstantiated type variable $tp")
         WildcardType
-      case FlexibleType(tp) => this(tp)
+      case tp: FlexibleType =>
+        this(tp.underlying)
       case tp: TypeProxy =>
         this(tp.underlying)
       case tp @ AndType(tp1, tp2) =>

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3410,6 +3410,11 @@ object Types {
 
   // --- FlexibleType -----------------------------------------------------------------
 
+  /* Represents a nullable type coming from Java code in a similar way to Platform Types
+   * in Kotlin. A FlexibleType(T) generally behaves like an abstract type with bad bounds
+   * T|Null .. T, so that T|Null <: FlexibleType(T) <: T.
+   */
+
   object FlexibleType {
     def apply(underlying: Type) = underlying match {
       case ft: FlexibleType => ft
@@ -3418,7 +3423,6 @@ object Types {
   }
   case class FlexibleType(underlying: Type) extends CachedGroundType with ValueType {
     def lo(using Context): Type = OrNull(underlying)
-    override def show(using Context) = i"FlexibleType($underlying)"
     def derivedFlexibleType(under: Type)(using Context): Type =
       if this.underlying eq under then this else FlexibleType(under)
     override def computeHash(bs: Binders): Int = doHash(bs, underlying)

compiler/src/dotty/tools/dotc/printing/PlainPrinter.scala

@@ -289,6 +289,8 @@ class PlainPrinter(_ctx: Context) extends Printer {
       case AnnotatedType(tpe, annot) =>
         if annot.symbol == defn.InlineParamAnnot || annot.symbol == defn.ErasedParamAnnot then toText(tpe)
         else toTextLocal(tpe) ~ " " ~ toText(annot)
+      case FlexibleType(tpe) =>
+        "FlexibleType(" ~ toText(tpe) ~ ")"
       case tp: TypeVar =>
         def toTextCaret(tp: Type) = if printDebug then toTextLocal(tp) ~ Str("^") else toText(tp)
         if (tp.isInstantiated)

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -564,6 +564,7 @@ private class ExtractAPICollector(using Context) extends ThunkHolder {
         val s = combineApiTypes(apiType(tp.tp1), apiType(tp.tp2))
         withMarker(s, orMarker)
       case tp: FlexibleType =>
+        // TODO: determine whether this is the right behaviour for sbt
         apiType(tp.underlying)
       case ExprType(resultType) =>
         withMarker(apiType(resultType), byNameMarker)