Treat `Function` literals uniformly, expecting SAM or FunctionN.

They both compile to INDY/MetaLambdaFactory, except when they
occur in a constructor call. (TODO: can we lift the ctor arg
expression to a method and avoid statically synthesizing
anonymous subclass altogether?)

Typers:
  - no longer synthesize SAMs -- *adapt* a Function literal
    to the expected (SAM/FunctionN) type
  - Deal with polymorphic/existential sams (relevant tests:
    pos/t8310, pos/t5099.scala, pos/t4869.scala) We know where
    to find the result type, as all Function nodes have a
    FunctionN-shaped type during erasure. (Including function
    literals targeting a SAM type -- the sam type is tracked as
    the *expected* type.)

    Lift restriction on sam types being class types. It's enough
    that they dealias to one, like regular instance creation
    expressions.

Contexts:
  - No longer need encl method hack for return in sam.

Erasure:
  - erasure preserves SAM type for function nodes
  - Normalize sam to erased function type during erasure,
    otherwise we may box the function body from `$anonfun(args)`
    to `{$anonfun(args); ()}` because the expected type for the
    body is now `Object`, and thus `Unit` does not conform.

Delambdafy:
  - must set static flag before calling createBoxingBridgeMethod
  - Refactored `createBoxingBridgeMethod` to wrap my head around
    boxing, reworked it to generalize from FunctionN's boxing
    needs to arbitrary LMF targets.

Other refactorings: ThisReferringMethodsTraverser, TreeGen.

1 files changed, 1 insertions, 4 deletions

diff --git a/test/files/neg/sammy_restrictions.check b/test/files/neg/sammy_restrictions.check
index 8cc49f9aa9..0276f3a067 100644
--- a/test/files/neg/sammy_restrictions.check
+++ b/test/files/neg/sammy_restrictions.check
@@ -8,9 +8,6 @@ sammy_restrictions.scala:32: error: type mismatch;
  required: TwoAbstract
   ((x: Int) => 0): TwoAbstract
             ^
-sammy_restrictions.scala:34: error: class type required but DerivedOneAbstract with OneAbstract found
-  ((x: Int) => 0): NonClassType                 // "class type required". I think we should avoid SAM translation here.
-            ^
 sammy_restrictions.scala:35: error: type mismatch;
  found   : Int => Int
  required: NoEmptyConstructor
@@ -46,4 +43,4 @@ sammy_restrictions.scala:44: error: type mismatch;
  required: PolyMethod
   ((x: Int) => 0): PolyMethod
             ^
-10 errors found
+9 errors found