Allow BCode to emit default interface methods

A DefDef owned by a trait may now have have a method body.
Such a method must be emitted without ACC_ABSTRACT, and
with the code attribute.

Tested by intercepting the compile pipeline and adding the
DEFAULTMETHOD flag and a method body before running
the backend.

Author: retronym

src/compiler/scala/tools/nsc/backend/icode/Members.scala

@@ -9,6 +9,7 @@ package backend
 package icode
 
 import scala.collection.{ mutable, immutable }
+import scala.reflect.internal.Flags
 import scala.reflect.internal.util.{ SourceFile, NoSourceFile }
 
 trait ReferenceEquality {
@@ -217,7 +218,7 @@ trait Members {
 
     /** Is this method deferred ('abstract' in Java sense)?
      */
-    def isAbstractMethod = symbol.isDeferred || symbol.owner.isInterface || native
+    def isAbstractMethod = symbol.isDeferred || (symbol.owner.isInterface && !symbol.hasFlag(Flags.DEFAULTMETHOD)) || native
 
     def isStatic: Boolean = symbol.isStaticMember
 

src/compiler/scala/tools/nsc/backend/jvm/BCodeSkelBuilder.scala

@@ -587,10 +587,10 @@ abstract class BCodeSkelBuilder extends BCodeHelpers {
       }
 
       val isNative         = methSymbol.hasAnnotation(definitions.NativeAttr)
-      val isAbstractMethod = (methSymbol.isDeferred || methSymbol.owner.isInterface)
+      val isAbstractMethod = (methSymbol.isDeferred || methSymbol.owner.isInterface) && !methSymbol.hasFlag(Flags.DEFAULTMETHOD)
       val flags = GenBCode.mkFlags(
         javaFlags(methSymbol),
-        if (claszSymbol.isInterface) asm.Opcodes.ACC_ABSTRACT   else 0,
+        if (isAbstractMethod)        asm.Opcodes.ACC_ABSTRACT   else 0,
         if (methSymbol.isStrictFP)   asm.Opcodes.ACC_STRICT     else 0,
         if (isNative)                asm.Opcodes.ACC_NATIVE     else 0  // native methods of objects are generated in mirror classes
       )

src/compiler/scala/tools/nsc/backend/jvm/BTypesFromSymbols.scala

@@ -560,7 +560,7 @@ class BTypesFromSymbols[G <: Global](val global: G) extends BTypes {
     import asm.Opcodes._
     GenBCode.mkFlags(
       if (privateFlag) ACC_PRIVATE else ACC_PUBLIC,
-      if (sym.isDeferred || sym.hasAbstractFlag) ACC_ABSTRACT else 0,
+      if ((sym.isDeferred && !sym.hasFlag(symtab.Flags.DEFAULTMETHOD))|| sym.hasAbstractFlag) ACC_ABSTRACT else 0,
       if (sym.isInterface) ACC_INTERFACE else 0,
       if (finalFlag && !sym.hasAbstractFlag) ACC_FINAL else 0,
       if (sym.isStaticMember) ACC_STATIC else 0,

src/compiler/scala/tools/nsc/backend/jvm/GenASM.scala

@@ -1391,10 +1391,11 @@ abstract class GenASM extends SubComponent with BytecodeWriters { self =>
       var resTpe: asm.Type = javaType(m.symbol.tpe.resultType)
       if (m.symbol.isClassConstructor)
         resTpe = asm.Type.VOID_TYPE
+      val isAbstractTraitMeth = isJInterface && !m.symbol.hasFlag(Flags.DEFAULTMETHOD)
 
       val flags = mkFlags(
         javaFlags(m.symbol),
-        if (isJInterface)          asm.Opcodes.ACC_ABSTRACT   else 0,
+        if (isAbstractTraitMeth)   asm.Opcodes.ACC_ABSTRACT   else 0,
         if (m.symbol.isStrictFP)   asm.Opcodes.ACC_STRICT     else 0,
         if (method.native)         asm.Opcodes.ACC_NATIVE     else 0, // native methods of objects are generated in mirror classes
         if(isDeprecated(m.symbol)) asm.Opcodes.ACC_DEPRECATED else 0  // ASM pseudo access flag

test/junit/scala/tools/nsc/backend/jvm/CodeGenTools.scala

@@ -93,6 +93,23 @@ object CodeGenTools {
     getGeneratedClassfiles(compiler.settings.outputDirs.getSingleOutput.get)
   }
 
+  def compileTransformed(compiler: Global)(scalaCode: String, javaCode: List[(String, String)] = Nil, beforeBackend: compiler.Tree => compiler.Tree): List[(String, Array[Byte])] = {
+    compiler.settings.stopBefore.value = "jvm" :: Nil
+    val run = newRun(compiler)
+    import compiler._
+    val scalaUnit = newCompilationUnit(scalaCode, "unitTestSource.scala")
+    val javaUnits = javaCode.map(p => newCompilationUnit(p._1, p._2))
+    val units = scalaUnit :: javaUnits
+    run.compileUnits(units, run.parserPhase)
+    compiler.settings.stopBefore.value = Nil
+    scalaUnit.body = beforeBackend(scalaUnit.body)
+    checkReport(compiler, _ => false)
+    val run1 = newRun(compiler)
+    run1.compileUnits(units, run1.phaseNamed("jvm"))
+    checkReport(compiler, _ => false)
+    getGeneratedClassfiles(compiler.settings.outputDirs.getSingleOutput.get)
+  }
+
   /**
    * Compile multiple Scala files separately into a single output directory.
    *

test/junit/scala/tools/nsc/backend/jvm/DefaultMethodTest.scala

@@ -0,0 +1,43 @@
+package scala.tools.nsc.backend.jvm
+
+import org.junit.Assert._
+import org.junit.Test
+
+import scala.collection.JavaConverters
+import scala.tools.asm.Opcodes
+import scala.tools.asm.tree.ClassNode
+import scala.tools.nsc.backend.jvm.CodeGenTools._
+import JavaConverters._
+import scala.tools.testing.ClearAfterClass
+
+object DefaultMethodTest extends ClearAfterClass.Clearable {
+  var compiler = newCompiler(extraArgs = "-Ybackend:GenBCode")
+  def clear(): Unit = { compiler = null }
+}
+
+class DefaultMethodTest extends ClearAfterClass {
+  ClearAfterClass.stateToClear = DirectCompileTest
+  val compiler = DirectCompileTest.compiler
+
+  @Test
+  def defaultMethodsViaGenBCode(): Unit = {
+    import compiler._
+    val code = "package pack { trait T { def foo: Int }}"
+    object makeFooDefaultMethod extends Transformer {
+      val Foo = TermName("foo")
+      /** Transforms a single tree. */
+      override def transform(tree: compiler.Tree): compiler.Tree = tree match {
+        case dd @ DefDef(_, Foo, _, _, _, _) =>
+          dd.symbol.setFlag(reflect.internal.Flags.DEFAULTMETHOD)
+          copyDefDef(dd)(rhs = Literal(Constant(1)).setType(definitions.IntTpe))
+        case _ => super.transform(tree)
+      }
+    }
+    val asmClasses: List[ClassNode] = readAsmClasses(compileTransformed(compiler)(code, Nil, makeFooDefaultMethod.transform(_)))
+    val foo = asmClasses.head.methods.iterator.asScala.toList.last
+    assertTrue("default method should not be abstract", (foo.access & Opcodes.ACC_ABSTRACT) == 0)
+    assertTrue("default method body emitted", foo.instructions.size() > 0)
+  }
+
+
+}