Add defaults to all trait interface methods.

Currently, these forward to the impl method in the impl class.
Later (perhaps after bootstrapping dependencies like partest)
we ought do away with the impl classes.

Author: retronym

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -1202,6 +1202,20 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
           } else {
             tree
           }
+        case dd @ DefDef(_, _, _, vparamss, _, EmptyTree) if sym.owner.isTrait =>
+          val isDeferred = enteringPhase(currentRun.erasurePhase.prev)(sym.isDeferred) // before lateDEFERRED
+          // TODO Duplicated with isImplementedStatically, at this point sym.owner is still the trait so the other method returns false.
+          val isImplementedStatically = sym.isMethod && !sym.isModule && !sym.hasFlag(ACCESSOR | SUPERACCESSOR)
+          if (!isDeferred && isImplementedStatically) {
+            val implSym = exitingMixin(implClass(sym.owner).info.member(sym.name))
+            sym.setFlag(Flags.DEFAULTMETHOD)
+            val tree = Apply(staticRef(implSym), gen.mkAttributedCast(gen.mkAttributedThis(sym.owner), sym.owner.typeOfThis) ::  vparamss.head.map(t => gen.mkAttributedRef(t.symbol)))
+            val app = exitingMixin(typedPos(tree.pos)(tree))
+            copyDefDef(dd)(rhs = app)
+          } else {
+            tree
+          }
+
         case Apply(Select(qual, _), args) =>
           /*  Changes `qual.m(args)` where m refers to an implementation
            *  class method to Q.m(S, args) where Q is the implementation module of

test/files/run/trait-defaults/A_1.scala

@@ -0,0 +1,3 @@
+trait A {
+  def a = "a"
+}

test/files/run/trait-defaults/B_2.java

@@ -0,0 +1,2 @@
+public class B_2 implements A {
+}

test/files/run/trait-defaults/Test_2.scala

@@ -0,0 +1,5 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    assert(new B_2().a == "a")
+  }
+}