Overhaul of Namers.

A digression into motivations:

It's not there yet but the future looks bright. I have winnowed the
number of mutation points down and I will take it down much further.
There are only a few fundamental state changes which take place and
these can be made to happen in a systematic fashion at well-known
junctions.

  1) Fresh symbols are allocated and (usually) assigned to a tree. 2)
  Symbol flags and access are manipulated. 3) A (possibly lazy) info is
  assigned to a symbol. 4) Synthetics are created or lazily positioned
  for creation

Among the complications is that the symbol's info often cannot be
determined in a straightforward fashion lest cycles develop. Type
inference is obviously dependent on having some type information, so the
black art is to pursue a) only the right information and b) only as far
as necessary to avoid cycles.

Compounding the difficulty is that synthetic methods must be introduced
before the typer phase. Over time a variety of ad-hoc mechanisms have
evolved to deal with these difficulties, and they have gotten us a good
distance (we have bean setter/getters, case classes, copy methods,
default getters, and more) but there are big disadvantages:

 - they are heavily custom fitted to the specific uses
 - because of the intertwingling of namer and typer, it is all
   only possible from the inside.  Compiler plugins are shut out.

A particularly difficult scenario has recently arisen with case classes.
They now receive a ProductN parent, but because a class's parents must
be completed before it can be completed, we encounter this:

  object Foo { type T = String }
  case class Foo(x: Foo.T, y: Foo.T) { }

Now one of class Foo's parents is Product2[T, T]. So class Foo cannot be
completed without information from object Foo. But object Foo needs to
be given these synthetic methods:

  def apply(x: T, y: T): Foo
  def unapply(x: Foo): Option[(T, T)]

You can see these two have their hands all over one another.

The good news is that I have established in principle that the problem
can be overcome, for that use and I think in a sufficiently general way
that plugins will be able to perform this kind of method synthesis,
with the following approach. For synthetic methods which require type
information before they can be created (this is the catch-22: once type
information is available, it's too late to add new synthetic methods) we
create a "stub symbol" like so:

  val sym = owner.newMethod("nameOfMethod")
  sym setInfo stubMethodInfo

stubMethodInfo will be some very general method type like Any* => Any
(or Nothing => Any, it really depends on how or whether it is used),
whatever it takes to pass type checking. At the same time, we enter the
stub symbol into a map along with a thunk which creates the symbol and
tree the way we would if we had full type information.

Then, immediately after a class is typed, the template is examined for
stub method symbols and when found, they are updated with the symbol
info found in the map, assigned to the associated tree, and added to the
class template. This approach will probably break down under some uses,
but I think it can take us a long way.

So all these changes in Namers are about evolving it to a point where I
can roll out a principled division of responsibility for those entities
which cannot be naively typed, and to unify the several different
approaches to lazy typing under a consistent and predictable mechanism.

If anyone would like to review this, please be my guest, but you might
want to wait one more commit.

1 files changed, 4 insertions, 1 deletions

diff --git a/test/files/neg/t1422.check b/test/files/neg/t1422.check
index 4db64f1d49..362d7ef36b 100644
--- a/test/files/neg/t1422.check
+++ b/test/files/neg/t1422.check
@@ -1,4 +1,7 @@
 t1422.scala:1: error: private[this] not allowed for case class parameters
 case class A(private[this] val foo:String) { }
                                ^
-one error found
+t1422.scala:1: error: value foo in class A cannot be accessed in A
+case class A(private[this] val foo:String) { }
+           ^
+two errors found