Fields phase

One step towards teasing apart the mixin phase, making
each phase that adds members to traits responsible for
mixing in those members into subclasses of said traits.

Another design tenet is to not emit symbols or trees
only to later remove them. Therefore, we model a
val in a trait as its accessor. The underlying field
is an implementation detail. It must be mixed into
subclasses, but has no business in a trait (an interface).

Also trying to reduce tree creation by changing less in subtrees
during tree transforms.

A lot of nice fixes fall out from this rework:

  - Correct bridges and more precise generic signatures for
    mixed in accessors, since they are now created before erasure.

  - Correct enclosing method attribute for classes nested in trait fields.

    Trait fields are now created as MethodSymbol (no longer TermSymbol).
    This symbol shows up in the `originalOwner` chain of a class declared
    within the field initializer. This promoted the field getter to
    being the enclosing method of the nested class, which it is not
    (the EnclosingMethod attribute is a source-level property).

  - Signature inference is now more similar between vals and defs

  - No more field for constant-typed vals, or mixed in accessors
    for subclasses. A constant val can be fully implemented in a trait.

TODO:
  - give same treatment to trait lazy vals (only accessors, no fields)
  - remove support for presuper vals in traits
    (they don't have the right init semantics in traits anyway)
  - lambdalift should emit accessors for captured vals in traits,
    not a field

Assorted notes from the full git history before squashing below.

Unit-typed vals: don't suppress field

it affects the memory model -- even a write of unit to a field is relevant...
unit-typed lazy vals should never receive a field
this need was unmasked by test/files/run/t7843-jsr223-service.scala,
which no longer printed the output expected from the `0 to 10 foreach`

Use getter.referenced to track traitsetter

reify's toolbox compiler changes the name of the trait
that owns the accessor between fields and constructors (`$` suffix),
so that the trait setter cannot be found when doing mkAssign in constructors
this could be solved by creating the mkAssign tree immediately during fields

anyway, first experiment: use `referenced` now that fields runs closer
to the constructors phase (I tried this before and something broke)

Infer result type for `val`s, like we do for `def`s

The lack of result type inference caused pos/t6780 to fail
in the new field encoding for traits, as there is no separate accessor,
and method synthesis computes the type signature based on the ValDef tree.
This caused a cyclic error in implicit search, because now the
implicit val's result type was not inferred from the super member,
and inferring it from the RHS would cause implicit search to consider
the member in question, so that a cycle is detected and type checking fails...

Regardless of the new encoding, we should consistently infer result types
for `def`s and `val`s.

Removed test/files/run/t4287inferredMethodTypes.scala and test/files/presentation/t4287c,
since they were relying on inferring argument types from "overridden" constructors
in a test for range positions of default arguments. Constructors don't override,
so that was a mis-feature of -Yinfer-argument-types.

Had to slightly refactor test/files/presentation/doc, as it was relying
on scalac inferring a big intersection type to approximate the anonymous
class that's instantiated for `override lazy val analyzer`.
Now that we infer `Global` as the expected type based on the overridden val,
we make `getComment` private in navigating between good old Skylla and Charybdis.
I'm not sure why we need this restriction for anonymous classes though;
only structural calls are restricted in the way that we're trying to avoid.

The old behavior is maintained nder -Xsource:2.11.

Tests:
  - test/files/{pos,neg}/val_infer.scala
  - test/files/neg/val_sig_infer_match.scala
  - test/files/neg/val_sig_infer_struct.scala

need NMT when inferring sig for accessor
Q: why are we calling valDefSig and not methodSig?
A: traits use defs for vals, but still use valDefSig...
keep accessor and field info in synch

1 files changed, 16 insertions, 15 deletions

diff --git a/test/files/neg/t6446-additional.check b/test/files/neg/t6446-additional.check
index e56a67b28b..45db63317c 100644
--- a/test/files/neg/t6446-additional.check
+++ b/test/files/neg/t6446-additional.check
@@ -10,18 +10,19 @@ superaccessors   6  add super accessors in traits and nested classes
        pickler   8  serialize symbol tables
      refchecks   9  reference/override checking, translate nested objects
        uncurry  10  uncurry, translate function values to anonymous classes
-     tailcalls  11  replace tail calls by jumps
-    specialize  12  @specialized-driven class and method specialization
- explicitouter  13  this refs to outer pointers
-       erasure  14  erase types, add interfaces for traits
-   posterasure  15  clean up erased inline classes
-      lazyvals  16  allocate bitmaps, translate lazy vals into lazified defs
-    lambdalift  17  move nested functions to top level
-  constructors  18  move field definitions into constructors
-       flatten  19  eliminate inner classes
-         mixin  20  mixin composition
-       cleanup  21  platform-specific cleanups, generate reflective calls
-    delambdafy  22  remove lambdas
-           jvm  23  generate JVM bytecode
-       ploogin  24  A sample phase that does so many things it's kind of hard...
-      terminal  25  the last phase during a compilation run
+        fields  11  synthesize accessors and fields
+     tailcalls  12  replace tail calls by jumps
+    specialize  13  @specialized-driven class and method specialization
+ explicitouter  14  this refs to outer pointers
+       erasure  15  erase types, add interfaces for traits
+   posterasure  16  clean up erased inline classes
+      lazyvals  17  allocate bitmaps, translate lazy vals into lazified defs
+    lambdalift  18  move nested functions to top level
+  constructors  19  move field definitions into constructors
+       flatten  20  eliminate inner classes
+         mixin  21  mixin composition
+       cleanup  22  platform-specific cleanups, generate reflective calls
+    delambdafy  23  remove lambdas
+           jvm  24  generate JVM bytecode
+       ploogin  25  A sample phase that does so many things it's kind of hard...
+      terminal  26  the last phase during a compilation run