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.

6 files changed, 33 insertions, 15 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
diff --git a/test/files/neg/sammy_restrictions.scala b/test/files/neg/sammy_restrictions.scala
index d003cfaf36..101342ad0b 100644
--- a/test/files/neg/sammy_restrictions.scala
+++ b/test/files/neg/sammy_restrictions.scala
@@ -31,7 +31,7 @@ object Test {
   (() => 0)      : NoAbstract
   ((x: Int) => 0): TwoAbstract
   ((x: Int) => 0): DerivedOneAbstract           // okay
-  ((x: Int) => 0): NonClassType                 // "class type required". I think we should avoid SAM translation here.
+  ((x: Int) => 0): NonClassType                 // okay -- we also allow type aliases in instantiation expressions, if they resolve to a class type
   ((x: Int) => 0): NoEmptyConstructor
   ((x: Int) => 0): OneEmptyConstructor          // okay
   ((x: Int) => 0): OneEmptySecondaryConstructor // derived class must have an empty *primary* to call.
diff --git a/test/files/pos/sammy_implicit.scala b/test/files/pos/sammy_implicit.scala
index c9c2519bab..e4b82df4cc 100644
--- a/test/files/pos/sammy_implicit.scala
+++ b/test/files/pos/sammy_implicit.scala
@@ -6,5 +6,5 @@ abstract class SamImplicitConvert {
 
   implicit def conv(xs: Array[Int]): Lst[Int]
 
-  val encoded = flatMap (_.getBytes)
+  def encoded = flatMap (_.getBytes)
 }
diff --git a/test/files/pos/sammy_poly.scala b/test/files/pos/sammy_poly.scala
index c629be7166..75ee36f654 100644
--- a/test/files/pos/sammy_poly.scala
+++ b/test/files/pos/sammy_poly.scala
@@ -1,7 +1,8 @@
 // test synthesizeSAMFunction where the sam type is not fully defined
 class T {
   trait F[T, U] { def apply(x: T): U }
+//  type F[T, U] = T => U
   // NOTE: the f(x) desugaring for now assumes the single abstract method is called 'apply'
   def app[T, U](x: T)(f: F[T, U]): U = f(x)
   app(1)(x => List(x))
-}
\ No newline at end of file
+}
diff --git a/test/files/run/indylambda-boxing/test.scala b/test/files/run/indylambda-boxing/test.scala
index cc0a460640..82f8d2f497 100644
--- a/test/files/run/indylambda-boxing/test.scala
+++ b/test/files/run/indylambda-boxing/test.scala
@@ -2,15 +2,16 @@ class Capture
 class Test {
   def test1 = (i: Int) => ""
   def test2 = (i: VC) => i
-  def test3 = (i: Int) => i
+  def test3 = (i: Int) => i // not adapted, specialized
 
-  def test4 = {val c = new Capture; (i: Int) => {(c, Test.this.toString); 42} }
+  def test4 = {val c = new Capture; (i: Int) => {(c, Test.this.toString); 42} } // not adapted, specialized
   def test5 = {val c = new Capture; (i: VC) => (c, Test.this.toString) }
   def test6 = {val c = new Capture; (i: Int) => (c, Test.this.toString) }
 
   def test7 = {val vc = new Capture; (i: Int) => vc }
-  def test8 = {val c = 42; (s: String) => (s, c)}
+  def test8 = {val c = 42; (s: String) => (s, c)} // not adapted
   def test9 = {val c = 42; (s: String) => ()}
+  def test10 = {(s: List[String]) => ()}
 }
 
 object Test {
diff --git a/test/junit/scala/tools/nsc/backend/jvm/IndyLambdaTest.scala b/test/junit/scala/tools/nsc/backend/jvm/IndyLambdaTest.scala
index 758566fe53..d29f6b0a13 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/IndyLambdaTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/IndyLambdaTest.scala
@@ -31,25 +31,44 @@ class IndyLambdaTest extends ClearAfterClass {
         case _ => Nil
       }.head
     }
+
+    val obj = "Ljava/lang/Object;"
+    val str = "Ljava/lang/String;"
+
     // unspecialized functions that have a primitive in parameter or return position
     // give rise to a "boxing bridge" method (which has the suffix `$adapted`).
     // This is because Scala's unboxing of null values gives zero, whereas Java's throw a NPE.
 
     // 1. Here we show that we are calling the boxing bridge (the lambda bodies here are compiled into
     //    methods of `(I)Ljava/lang/Object;` / `(I)Ljava/lang/Object;` respectively.)
-    assertEquals("(Ljava/lang/Object;)Ljava/lang/Object;", implMethodDescriptorFor("(x: Int) => new Object"))
-    assertEquals("(Ljava/lang/Object;)Ljava/lang/Object;", implMethodDescriptorFor("(x: Object) => 0"))
+    assertEquals(s"($obj)$obj", implMethodDescriptorFor("(x: Int) => new Object"))
+    assertEquals(s"($obj)$obj", implMethodDescriptorFor("(x: Object) => 0"))
 
     // 2a. We don't need such adaptations for parameters or return values with types that differ
     // from Object due to other generic substitution, LambdaMetafactory will downcast the arguments.
-    assertEquals("(Ljava/lang/String;)Ljava/lang/String;", implMethodDescriptorFor("(x: String) => x"))
+    assertEquals(s"($str)$str", implMethodDescriptorFor("(x: String) => x"))
 
     // 2b. Testing 2a. in combination with 1.
-    assertEquals("(Ljava/lang/Object;)Ljava/lang/String;", implMethodDescriptorFor("(x: Int) => \"\""))
-    assertEquals("(Ljava/lang/String;)Ljava/lang/Object;", implMethodDescriptorFor("(x: String) => 0"))
+    assertEquals(s"($obj)$str", implMethodDescriptorFor("(x: Int) => \"\""))
+    assertEquals(s"($str)$obj", implMethodDescriptorFor("(x: String) => 0"))
 
     // 3. Specialized functions, don't need any of this as they implement a method like `apply$mcII$sp`,
     //    and the (un)boxing is handled in the base class in code emitted by scalac.
     assertEquals("(I)I", implMethodDescriptorFor("(x: Int) => x"))
+
+    // non-builtin sams are like specialized functions
+    compileClasses(compiler)("class VC(private val i: Int) extends AnyVal; trait FunVC { def apply(a: VC): VC }")
+    assertEquals("(I)I", implMethodDescriptorFor("((x: VC) => x): FunVC"))
+
+    compileClasses(compiler)("trait Fun1[T, U] { def apply(a: T): U }")
+    assertEquals(s"($obj)$str", implMethodDescriptorFor("(x => x.toString): Fun1[Int, String]"))
+    assertEquals(s"($obj)$obj", implMethodDescriptorFor("(x => println(x)): Fun1[Int, Unit]"))
+    assertEquals(s"($obj)$str", implMethodDescriptorFor("((x: VC) => \"\") : Fun1[VC, String]"))
+    assertEquals(s"($str)$obj", implMethodDescriptorFor("((x: String) => new VC(0)) : Fun1[String, VC]"))
+
+    compileClasses(compiler)("trait Coll[A, Repr] extends Any")
+    compileClasses(compiler)("final class ofInt(val repr: Array[Int]) extends AnyVal with Coll[Int, Array[Int]]")
+
+    assertEquals(s"([I)$obj", implMethodDescriptorFor("((xs: Array[Int]) => new ofInt(xs)): Array[Int] => Coll[Int, Array[Int]]"))
   }
 }