Only allow opaque type def at outermost

[Cherry-picked 16ebbea]

Author: noti0na1

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

@@ -4377,7 +4377,7 @@ object Parsers {
      *                 | Expr1
      *                 |
      */
-    def blockStatSeq(): List[Tree] = checkNoEscapingPlaceholders {
+    def blockStatSeq(outermost: Boolean = false): List[Tree] = checkNoEscapingPlaceholders {
       val stats = new ListBuffer[Tree]
       while
         var empty = false
@@ -4389,7 +4389,7 @@ object Parsers {
           stats += closure(in.offset, Location.InBlock, modifiers(BitSet(IMPLICIT)))
         else if isIdent(nme.extension) && followingIsExtension() then
           stats += extension()
-        else if ctx.mode.is(Mode.Interactive) && isDefIntro(localModifierTokens) then
+        else if outermost && ctx.mode.is(Mode.Interactive) && isDefIntro(localModifierTokens) then
           stats +++= localDef(in.offset)
         else if isDefIntro(localModifierTokens, excludedSoftModifiers = Set(nme.`opaque`)) then
           stats +++= localDef(in.offset)

compiler/src/dotty/tools/repl/ParseResult.scala

@@ -122,7 +122,7 @@ object ParseResult {
 
   private def parseStats(using Context): List[untpd.Tree] = {
     val parser = new Parser(ctx.source)
-    val stats = parser.blockStatSeq()
+    val stats = parser.blockStatSeq(outermost = true)
     parser.accept(Tokens.EOF)
     stats
   }