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, 5 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/CleanUp.scala b/src/compiler/scala/tools/nsc/transform/CleanUp.scala
index 0fb6213d36..81df28bc87 100644
--- a/src/compiler/scala/tools/nsc/transform/CleanUp.scala
+++ b/src/compiler/scala/tools/nsc/transform/CleanUp.scala
@@ -456,6 +456,11 @@ abstract class CleanUp extends Statics with Transform with ast.TreeDSL {
 
         super.transform(treeCopy.ApplyDynamic(tree, atPos(fn.pos)(Ident(SymbolLiteral_dummy).setType(SymbolLiteral_dummy.info)), LIT(SymbolLiteral_bootstrap) :: arg :: Nil))
 
+      // Drop the TypeApply, which was used in Erasure to make `synchronized { ... } ` erase like `...`
+      // (and to avoid boxing the argument to the polymorphic `synchronized` method).
+      case app@Apply(TypeApply(fun, _), args) if fun.symbol == Object_synchronized =>
+        super.transform(treeCopy.Apply(app, fun, args))
+
       // Replaces `Array(Predef.wrapArray(ArrayValue(...).$asInstanceOf[...]), <tag>)`
       // with just `ArrayValue(...).$asInstanceOf[...]`
       //