Specialize erasure of `synchronized` primitive method

The goal is to avoid emitting unneeded `BoxedUnit` values,
which are the result of adapting a `Unit`-typed expression
inside a `synchronized(...)` to the erased type of
`synchronized`'s argument -- `Object`.

The proposed solution gives `synchronized` a polymorphic
type (the info of the type param is still erased so that
bounds checking works in the erased type system), so that
an application `synchronized(println("boo"))` erases to
`synchronized[Unit])(println("boo"))`, and no boxing is
performed on the `println("boo")` argument, whose expected
type is now `Unit` instead of `Object`.

1 files changed, 2 insertions, 8 deletions

diff --git a/test/files/run/t6028.check b/test/files/run/t6028.check
index f757bc93ff..d6cc452bbf 100644
--- a/test/files/run/t6028.check
+++ b/test/files/run/t6028.check
@@ -56,14 +56,8 @@ package <empty> {
     };
     final <stable> private[this] def MethodLocalObject$1(barParam$1: Int, MethodLocalObject$module$1: runtime.VolatileObjectRef): T#MethodLocalObject$2.type = {
       if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
-        {
-          T.this.synchronized({
-            if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
-              MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1);
-            scala.runtime.BoxedUnit.UNIT
-          });
-          ()
-        };
+        T.this.synchronized[Unit](if (MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]().eq(null))
+          MethodLocalObject$module$1.elem = new T#MethodLocalObject$2.type(T.this, barParam$1));
       MethodLocalObject$module$1.elem.$asInstanceOf[T#MethodLocalObject$2.type]()
     };
     @SerialVersionUID(value = 0) final <synthetic> class $anonfun$tryy$1 extends scala.runtime.AbstractFunction0$mcV$sp with Serializable {