Add comment explaining why remove variance, use Name#startsWith ins…

…tead of `.lastPart.startsWith`

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1448,7 +1448,11 @@ object Parsers {
     }
 
     private def makeKindProjectorTypeDef(name: TypeName): TypeDef = {
-      val isVarianceAnnotated = name.lastPart.startsWith("+") || name.lastPart.startsWith("-")
+      val isVarianceAnnotated = name.startsWith("+") || name.startsWith("-")
+      // We remove the variance marker from the name without passing along the specified variance at all
+      // The real variance will be inferred at a later stage but may contradict the variance specified,
+      // This is ok, because `-Ykind-projector` is for cross-compiling existing Scala 2 code, not for writing new code,
+      // we may assume that variance annotations have already been checked by the Scala 2 compiler.
       val unannotatedName = if (isVarianceAnnotated) name.mapLast(_.drop(1)) else name
       TypeDef(unannotatedName, WildcardTypeBoundsTree()).withFlags(Param)
     }