SI-9393 fix modifiers of ClassBTypes for Java annotations

The Scala classfile and java source parsers make Java annotation
classes (which are actually interfaces at the classfile level) look
like Scala annotation classes:
  - the INTERFACE / ABSTRACT flags are not added
  - scala.annotation.Annotation is added as superclass
  - scala.annotation.ClassfileAnnotation is added as interface

This makes type-checking @Annot uniform, whether it is defined in Java
or Scala.

This is a hack that leads to various bugs (SI-9393, SI-9400). Instead
the type-checking should be special-cased for Java annotations.

This commit fixes SI-9393 and a part of SI-9400, but it's still easy
to generate invalid classfiles. Restores the assertions that were
disabled in #4621. I'd like to leave these assertions in: they
are valuable and helped uncovering the issue being fixed here.

A new flag JAVA_ANNOTATION is introduced for Java annotation
ClassSymbols, similar to the existing ENUM flag. When building
ClassBTypes for Java annotations, the flags, superclass and interfaces
are recovered to represent the situation in the classfile.

Cleans up and documents the flags space in the area of "late" and
"anti" flags.

The test for SI-9393 is extended to test both the classfile and the
java source parser.

5 files changed, 42 insertions, 20 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeAsmCommon.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeAsmCommon.scala
index daa31c5dfe..23cb170b38 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeAsmCommon.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeAsmCommon.scala
@@ -256,6 +256,9 @@ final class BCodeAsmCommon[G <: Global](val global: G) {
       if (hasAbstractMethod) ACC_ABSTRACT else 0
     }
     GenBCode.mkFlags(
+      // SI-9393: the classfile / java source parser make java annotation symbols look like classes.
+      // here we recover the actual classfile flags.
+      if (classSym.hasJavaAnnotationFlag)                    ACC_ANNOTATION | ACC_INTERFACE | ACC_ABSTRACT else 0,
       if (classSym.isPublic)                                 ACC_PUBLIC    else 0,
       if (classSym.isFinal)                                  ACC_FINAL     else 0,
       // see the link above. javac does the same: ACC_SUPER for all classes, but not interfaces.
@@ -310,10 +313,10 @@ final class BCodeAsmCommon[G <: Global](val global: G) {
   }
 
   private def retentionPolicyOf(annot: AnnotationInfo): Symbol =
-    annot.atp.typeSymbol.getAnnotation(AnnotationRetentionAttr).map(_.assocs).map(assoc =>
+    annot.atp.typeSymbol.getAnnotation(AnnotationRetentionAttr).map(_.assocs).flatMap(assoc =>
       assoc.collectFirst {
         case (`nme`.value, LiteralAnnotArg(Constant(value: Symbol))) => value
-      }).flatten.getOrElse(AnnotationRetentionPolicyClassValue)
+      }).getOrElse(AnnotationRetentionPolicyClassValue)
 
   def implementedInterfaces(classSym: Symbol): List[Symbol] = {
     // Additional interface parents based on annotations and other cues
@@ -322,9 +325,18 @@ final class BCodeAsmCommon[G <: Global](val global: G) {
       case _          => None
     }
 
-    def isInterfaceOrTrait(sym: Symbol) = sym.isInterface || sym.isTrait
+    // SI-9393: java annotations are interfaces, but the classfile / java source parsers make them look like classes.
+    def isInterfaceOrTrait(sym: Symbol) = sym.isInterface || sym.isTrait || sym.hasJavaAnnotationFlag
 
-    val allParents = classSym.info.parents ++ classSym.annotations.flatMap(newParentForAnnotation)
+    val classParents = {
+      val parents = classSym.info.parents
+      // SI-9393: the classfile / java source parsers add Annotation and ClassfileAnnotation to the
+      // parents of a java annotations. undo this for the backend (where we need classfile-level information).
+      if (classSym.hasJavaAnnotationFlag) parents.filterNot(c => c.typeSymbol == ClassfileAnnotationClass || c.typeSymbol == AnnotationClass)
+      else parents
+    }
+
+    val allParents = classParents ++ classSym.annotations.flatMap(newParentForAnnotation)
 
     // We keep the superClass when computing minimizeParents to eliminate more interfaces.
     // Example: T can be eliminated from D
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
index c7c2e43f82..a9b6a312e9 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala
@@ -153,9 +153,9 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
      */
     private def initJClass(jclass: asm.ClassVisitor) {
 
-      val ps = claszSymbol.info.parents
-      val superClass: String = if (ps.isEmpty) ObjectReference.internalName else internalName(ps.head.typeSymbol)
-      val interfaceNames = classBTypeFromSymbol(claszSymbol).info.get.interfaces map {
+      val bType = classBTypeFromSymbol(claszSymbol)
+      val superClass = bType.info.get.superClass.getOrElse(ObjectReference).internalName
+      val interfaceNames = bType.info.get.interfaces map {
         case classBType =>
           if (classBType.isNestedClass.get) { innerClassBufferASM += classBType }
           classBType.internalName
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala b/src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala
index 9bae63f1fc..8720da84e8 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BTypes.scala
@@ -841,17 +841,16 @@ abstract class BTypes {
 
       assert(!ClassBType.isInternalPhantomType(internalName), s"Cannot create ClassBType for phantom type $this")
 
-      // TODO bring these back in a way that doesn't trip pos/t9393
-      // assert(
-      //   if (info.get.superClass.isEmpty) { isJLO(this) || (isCompilingPrimitive && ClassBType.hasNoSuper(internalName)) }
-      //   else if (isInterface.get) isJLO(info.get.superClass.get)
-      //   else !isJLO(this) && ifInit(info.get.superClass.get)(!_.isInterface.get),
-      //   s"Invalid superClass in $this: ${info.get.superClass}"
-      // )
-      // assert(
-      //   info.get.interfaces.forall(c => ifInit(c)(_.isInterface.get)),
-      //   s"Invalid interfaces in $this: ${info.get.interfaces}"
-      // )
+      assert(
+        if (info.get.superClass.isEmpty) { isJLO(this) || (isCompilingPrimitive && ClassBType.hasNoSuper(internalName)) }
+        else if (isInterface.get) isJLO(info.get.superClass.get)
+        else !isJLO(this) && ifInit(info.get.superClass.get)(!_.isInterface.get),
+        s"Invalid superClass in $this: ${info.get.superClass}"
+      )
+      assert(
+        info.get.interfaces.forall(c => ifInit(c)(_.isInterface.get)),
+        s"Invalid interfaces in $this: ${info.get.interfaces}"
+      )
 
       assert(info.get.nestedClasses.forall(c => ifInit(c)(_.isNestedClass.get)), info.get.nestedClasses)
     }
diff --git a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
index 9b4451d492..8ded58d3d9 100644
--- a/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
+++ b/src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala
@@ -216,7 +216,18 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
   }
 
   private def setClassInfo(classSym: Symbol, classBType: ClassBType): ClassBType = {
-    val superClassSym = if (classSym.isImplClass) ObjectClass else classSym.superClass
+    // Check for isImplClass: trait implementation classes have NoSymbol as superClass
+    // Check for hasAnnotationFlag for SI-9393: the classfile / java source parsers add
+    // scala.annotation.Annotation as superclass to java annotations. In reality, java
+    // annotation classfiles have superclass Object (like any interface classfile).
+    val superClassSym = if (classSym.isImplClass || classSym.hasJavaAnnotationFlag) ObjectClass else {
+      val sc = classSym.superClass
+      // SI-9393: Java annotation classes don't have the ABSTRACT/INTERFACE flag, so they appear
+      // (wrongly) as superclasses. Fix this for BTypes: the java annotation will appear as interface
+      // (handled by method implementedInterfaces), the superclass is set to Object.
+      if (sc.hasJavaAnnotationFlag) ObjectClass
+      else sc
+    }
     assert(
       if (classSym == ObjectClass)
         superClassSym == NoSymbol
diff --git a/src/compiler/scala/tools/nsc/javac/JavaParsers.scala b/src/compiler/scala/tools/nsc/javac/JavaParsers.scala
index 03f0236734..67921303b9 100644
--- a/src/compiler/scala/tools/nsc/javac/JavaParsers.scala
+++ b/src/compiler/scala/tools/nsc/javac/JavaParsers.scala
@@ -751,7 +751,7 @@ trait JavaParsers extends ast.parser.ParsersCommon with JavaScanners {
       val (statics, body) = typeBody(AT, name)
       val templ = makeTemplate(annotationParents, body)
       addCompanionObject(statics, atPos(pos) {
-        ClassDef(mods, name, List(), templ)
+        ClassDef(mods | Flags.JAVA_ANNOTATION, name, List(), templ)
       })
     }