Consider the following code:

trait Base {
  def foo(p: Int => Boolean): Boolean
}

trait Impl extends Base {
  def foo(p: Int => Boolean): Boolean = p(1)
}

trait ImplOptimized extends Impl {
  @inline
  override final def foo(p: Int => Boolean): Boolean = p(1)
}

class Foo extends Base with ImplOptimized

class Bar {
  def bar(f: Foo) = f.foo(x => x == 1)
}

When compiled with -optimize -Yinline-warnings I get the following:
Test.scala:17: warning: Could not inline required method foo because callee (ImplOptimized.foo) has no code.
def bar(f: Foo) = f.foo(x => x == 1)

I asked Iulian about this and he said it might be tricky but should be possible to get inlining to work here. First we would need inline the forwarder Foo.foo that forwards to ImplOptimized.foo and then inline that method (with little twist related to handling $this).

Now, why getting this to work is important? Because we could mark all methods in LinearSeqOptimized as final and @inline and get rid of closure allocations for many operations on collections like List. This would be huge gain for the compiler.