Try using flagsUNSAFE (thanks @smarter)

Author: liufengyun

compiler/src/dotty/tools/dotc/core/classfile/ClassfileParser.scala

@@ -628,19 +628,11 @@ class ClassfileParser(
   }
 
   class AttributeCompleter(sym: Symbol) {
-    var codeAttribute: Boolean = false
     var sig: String = null
     var constant: Constant = null
     var exceptions: List[NameOrString] = Nil
     var annotations: List[Annotation] = Nil
     def complete(tp: Type, isVarargs: Boolean = false)(using Context): Type = {
-        // flag test will trigger completion and cycles, thus have to be lazy
-      if (codeAttribute && sym.owner.isAllOf(Flags.JavaInterface)) {
-        sym.resetFlag(Flags.Deferred)
-        sym.owner.resetFlag(Flags.PureInterface)
-        report.log(s"$sym in ${sym.owner} is a java 8+ default method.")
-      }
-
       val updatedType =
         if sig == null then tp
         else {
@@ -715,7 +707,12 @@ class ClassfileParser(
 
         case tpnme.CodeATTR =>
           in.skip(attrLen)
-          res.codeAttribute = true
+          // flag test will trigger completion and cycles, thus have to be lazy
+          if (sym.owner.flagsUNSAFE.isAllOf(Flags.JavaInterface)) {
+            sym.resetFlag(Flags.Deferred)
+            sym.owner.resetFlag(Flags.PureInterface)
+            report.log(s"$sym in ${sym.owner} is a java 8+ default method.")
+          }
 
         case _ =>
       }