Move @static immutable fields & methods to companion traits

The original code in addition adds the flag `Mutable` while moving,
which does not respect Scala semantics.

Author: liufengyun

compiler/src/dotty/tools/dotc/transform/MoveStatics.scala

@@ -24,12 +24,11 @@ class MoveStatics extends MiniPhase with SymTransformer {
   override def phaseName: String = MoveStatics.name
 
   def transformSym(sym: SymDenotation)(implicit ctx: Context): SymDenotation = {
-    if (sym.hasAnnotation(defn.ScalaStaticAnnot) && sym.owner.is(Flags.Module) &&
-        sym.owner.companionClass.exists && !sym.owner.companionClass.is(Flags.Trait)) {
+    if (sym.hasAnnotation(defn.ScalaStaticAnnot) && sym.owner.is(Flags.Module) && sym.owner.companionClass.exists &&
+        (sym.is(Flags.Method) || !(sym.is(Flags.Mutable) && sym.owner.companionClass.is(Flags.Trait)))) {
       sym.owner.asClass.delete(sym.symbol)
       sym.owner.companionClass.asClass.enter(sym.symbol)
-      val flags = if (sym.is(Flags.Method)) sym.flags else sym.flags | Flags.Mutable
-      sym.copySymDenotation(owner = sym.owner.companionClass, initFlags = flags)
+      sym.copySymDenotation(owner = sym.owner.companionClass)
     }
     else sym
   }
@@ -59,9 +58,8 @@ class MoveStatics extends MiniPhase with SymTransformer {
       }
 
       def move(module: TypeDef, companion: TypeDef): List[Tree] = {
-        assert(companion ne module)
+        assert(companion != module)
         if (!module.symbol.is(Flags.Module)) move(companion, module)
-        else if (companion != null && companion.symbol.is(Flags.Trait)) List(module, companion)
         else {
           val allMembers =
             (if(companion != null) {companion.rhs.asInstanceOf[Template].body} else Nil) ++

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTest.scala

@@ -73,6 +73,10 @@ trait DottyBytecodeTest {
     classNode.methods.asScala.find(_.name == name) getOrElse
       sys.error(s"Didn't find method '$name' in class '${classNode.name}'")
 
+  protected def getField(classNode: ClassNode, name: String): FieldNode =
+    classNode.fields.asScala.find(_.name == name) getOrElse
+      sys.error(s"Didn't find field '$name' in class '${classNode.name}'")
+
   def diffInstructions(isa: List[Instruction], isb: List[Instruction]): String = {
     val len = Math.max(isa.length, isb.length)
     val sb = new StringBuilder

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -695,4 +695,50 @@ class TestBCode extends DottyBytecodeTest {
         "`test` was not properly generated\n" + diffInstructions(instructions, expected))
     }
   }
+
+  @Test def i5924b = {
+    val source =
+      """|import scala.annotation.static
+         |trait Base
+         |
+         |object Base {
+         |  @static val x = 10
+         |  @static final val y = 10
+         |  @static def f: Int = 30
+         |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn   = dir.lookupName("Base.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+      getMethod(clsNode, "f")
+      getField(clsNode, "x")
+      getField(clsNode, "y")
+    }
+  }
+
+  @Test def i5924c = {
+    val source =
+      """|import scala.annotation.static
+         |class Base
+         |
+         |object Base {
+         |  @static val x = 10
+         |  @static final val y = 10
+         |  @static var a = 10
+         |  @static final var b = 10
+         |  @static def f: Int = 30
+         |}
+      """.stripMargin
+
+    checkBCode(source) { dir =>
+      val clsIn   = dir.lookupName("Base.class", directory = false).input
+      val clsNode = loadClassNode(clsIn)
+      getMethod(clsNode, "f")
+      getField(clsNode, "x")
+      getField(clsNode, "y")
+      getField(clsNode, "a")
+      getField(clsNode, "b")
+    }
+  }
 }

tests/run/i5924b.scala

@@ -0,0 +1,13 @@
+import scala.annotation.static
+
+trait Base
+
+object Base {
+  @static val x = 10
+  @static final val y = 10
+  @static def f: Int = 30
+}
+
+object Test {
+  def main(args: Array[String]): Unit = Base
+}

tests/run/i5924c.scala

@@ -0,0 +1,15 @@
+import scala.annotation.static
+
+class Base
+
+object Base {
+  @static val x = 10
+  @static final val y = 10
+  @static var a = 10
+  @static final var b = 10
+  @static def f: Int = 30
+}
+
+object Test {
+  def main(args: Array[String]): Unit = Base
+}