SI-10009 Fields survive untypecheck/retypecheck

Some places in the compiler, and many places in macros, use
`untypecheck` (aka `resetAttrs`) to strip types and local symbols
from a tree before retypechecking it under some different context.

The refactoring of the desugaring of vals and vars in Scala 2.12.0
broke an assumption in this facility.

When a ValDef must be split into multiple members (e.g. a field and
a getter, or a perhaps also a setter), the ValDef that was parsed
assumes the role of the `field`, and the trees for other members are
stached by `Namer` to the `synthetics` map of the compilation unit,
in order to spliced into the right statement list by typechecking.
See `enterGetterSetter` for more details.

However, the parsed ValDef is now used verbatim, carrying the meaning
(ie, the symbol) of the `private[this]` field. This tree now had
an inconsistency between the flags in `tree.mods.flags` and
`tree.symbol.flags`. `tree.name` also differed from `tree.symbol.name`
(the latter was renamed to be a local name, ie one with a trailing space.)

When `ResetAttrs` stripped off the symbol and we retypechecked, we'd
end up with two symbols in scope with the same name.

In the first from the `run` test:

```
================================================================================
{
  class a extends scala.AnyRef {
    def <init>(): a = {
      a.super.<init>();
      ()
    };
    private[this] val x: Int = 42;
    <stable> <accessor> def x: Int = a.this.x
  };
  new a()
}

{
  class a extends scala.AnyRef {
    def <init>() = {
      super.<init>();
      ()
    };
    val x = 42; // oops, the name is "x" rather than "x " and we've missing `private[this]`!
    <stable> <accessor> def x: Int = a.this.x
  };
  new a()
}

scala.tools.reflect.ToolBoxError: reflective typecheck has failed: x is already defined as value x
```

This commit uses the flags and name of the symbol in `typedValDef`.
I've also had to modify the internals of `CodePrinter` to use the
implicit, override, and deferred flags from the modifiers of an
accessor when recovering pre-typer tree for a ValDef.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
index d11417192d..0f257d3717 100644
--- a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
@@ -132,7 +132,11 @@ trait MethodSynthesis {
 
       // only one symbol can have `tree.pos`, the others must focus their position
       // normally the field gets the range position, but if there is none, give it to the getter
+      //
+      // SI-10009 the tree's modifiers can be temporarily out of sync with the new symbol's flags.
+      //          typedValDef corrects this later on.
       tree.symbol = fieldSym orElse (getterSym setPos tree.pos)
+
       val namer = namerOf(tree.symbol)
 
       // the valdef gets the accessor symbol for a lazy val (too much going on in its RHS)