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Until now, concrete methods in traits were encoded with
"trait implementation classes".
- Such a trait would compile to two class files
- the trait interface, a Java interface, and
- the implementation class, containing "trait implementation methods"
- trait implementation methods are static methods has an explicit self
parameter.
- some methods don't require addition of an interface method, such as
private methods. Calls to these directly call the implementation method
- classes that mixin a trait install "trait forwarders", which implement
the abstract method in the interface by forwarding to the trait
implementation method.
The new encoding:
- no longer emits trait implementation classes or trait implementation
methods.
- instead, concrete methods are simply retained in the interface, as JVM 8
default interface methods (the JVM spec changes in
[JSR-335](http://download.oracle.com/otndocs/jcp/lambda-0_9_3-fr-eval-spec/index.html)
pave the way)
- use `invokespecial` to call private or particular super implementations
of a method (rather `invokestatic`)
- in cases when we `invokespecial` to a method in an indirect ancestor, we add
that ancestor redundantly as a direct parent. We are investigating alternatives
approaches here.
- we still emit trait fowrarders, although we are
[investigating](https://github.com/scala/scala-dev/issues/98) ways to only do
this when the JVM would be unable to resolve the correct method using its rules
for default method resolution.
Here's an example:
```
trait T {
println("T")
def m1 = m2
private def m2 = "m2"
}
trait U extends T {
println("T")
override def m1 = super[T].m1
}
class C extends U {
println("C")
def test = m1
}
```
The old and new encodings are displayed and diffed here: https://gist.github.com/retronym/f174d23f859f0e053580
Some notes in the implementation:
- No need to filter members from class decls at all in AddInterfaces
(although we do have to trigger side effecting info transformers)
- We can now emit an EnclosingMethod attribute for classes nested
in private trait methods
- Created a factory method for an AST shape that is used in
a number of places to symbolically bind to a particular
super method without needed to specify the qualifier of
the `Super` tree (which is too limiting, as it only allows
you to refer to direct parents.)
- I also found a similar tree shape created in Delambdafy,
that is better expressed with an existing tree creation
factory method, mkSuperInit.
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