[sammy] eta-expansion, overloading (SI-8310)

Playing with Java 8 Streams from the repl showed
we weren't eta-expanding, nor resolving overloading for SAMs.

Also, the way Java uses wildcards to represent use-site variance
stresses type inference past its bendiness point (due to excessive existentials).

I introduce `wildcardExtrapolation` to simplify the resulting types
(without losing precision): `wildcardExtrapolation(tp) =:= tp`.

For example, the `MethodType` given by `def bla(x: (_ >: String)): (_ <: Int)`
is both a subtype and a supertype of `def bla(x: String): Int`.

Translating http://winterbe.com/posts/2014/07/31/java8-stream-tutorial-examples/
into Scala shows most of this works, though we have some more work to do (see near the end).

```
scala> import java.util.Arrays
scala> import java.util.stream.Stream
scala> import java.util.stream.IntStream
scala> val myList = Arrays.asList("a1", "a2", "b1", "c2", "c1")
myList: java.util.List[String] = [a1, a2, b1, c2, c1]

scala> myList.stream.filter(_.startsWith("c")).map(_.toUpperCase).sorted.forEach(println)
C1
C2

scala> myList.stream.filter(_.startsWith("c")).map(_.toUpperCase).sorted
res8: java.util.stream.Stream[?0] = java.util.stream.SortedOps$OfRef@133e7789

scala> Arrays.asList("a1", "a2", "a3").stream.findFirst.ifPresent(println)
a1

scala> Stream.of("a1", "a2", "a3").findFirst.ifPresent(println)
a1

scala> IntStream.range(1, 4).forEach(println)
<console>:37: error: object creation impossible, since method accept in trait IntConsumer of type (x$1: Int)Unit is not defined
(Note that Int does not match Any: class Int in package scala is a subclass of class Any in package scala, but method parameter types must match exactly.)
              IntStream.range(1, 4).forEach(println)
                                            ^

scala> IntStream.range(1, 4).forEach(println(_: Int)) // TODO: can we avoid this annotation?
1
2
3

scala> Arrays.stream(Array(1, 2, 3)).map(n => 2 * n + 1).average.ifPresent(println(_: Double))
5.0

scala> Stream.of("a1", "a2", "a3").map(_.substring(1)).mapToInt(_.parseInt).max.ifPresent(println(_: Int)) // whoops!
ReplGlobal.abort: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
error: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
scala.reflect.internal.FatalError: Unknown type: <error>, <error> [class scala.reflect.internal.Types$ErrorType$, class scala.reflect.internal.Types$ErrorType$] TypeRef? false
	at scala.reflect.internal.Reporting$class.abort(Reporting.scala:59)

scala> IntStream.range(1, 4).mapToObj(i => "a" + i).forEach(println)
a1
a2
a3

```

1 files changed, 26 insertions, 10 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 70acb03584..c41b81aaf5 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -741,6 +741,26 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       case _ =>
     }
 
+    /**
+     * Convert a SAM type to the corresponding FunctionType,
+     * extrapolating BoundedWildcardTypes in the process
+     * (no type precision is lost by the extrapolation,
+     *  but this facilitates dealing with the types arising from Java's use-site variance).
+     */
+    def samToFunctionType(tp: Type, sam: Symbol = NoSymbol): Type = {
+      val samSym = sam orElse samOf(tp)
+
+      def correspondingFunctionSymbol = {
+        val numVparams = samSym.info.params.length
+        if (numVparams > definitions.MaxFunctionArity) NoSymbol
+        else FunctionClass(numVparams)
+      }
+
+      if (samSym.exists && samSym.owner != correspondingFunctionSymbol) // don't treat Functions as SAMs
+        wildcardExtrapolation(normalize(tp memberInfo samSym))
+      else NoType
+    }
+
     /** Perform the following adaptations of expression, pattern or type `tree` wrt to
      *  given mode `mode` and given prototype `pt`:
      *  (-1) For expressions with annotated types, let AnnotationCheckers decide what to do
@@ -824,7 +844,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           case Block(_, tree1) => tree1.symbol
           case _               => tree.symbol
         }
-        if (!meth.isConstructor && isFunctionType(pt)) { // (4.2)
+        if (!meth.isConstructor && (isFunctionType(pt) || samOf(pt).exists)) { // (4.2)
           debuglog(s"eta-expanding $tree: ${tree.tpe} to $pt")
           checkParamsConvertible(tree, tree.tpe)
           val tree0 = etaExpand(context.unit, tree, this)
@@ -2838,7 +2858,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
        * as `(a => a): Int => Int` should not (yet) get the sam treatment.
        */
       val sam =
-        if (!settings.Xexperimental || pt.typeSymbol == FunctionSymbol) NoSymbol
+        if (pt.typeSymbol == FunctionSymbol) NoSymbol
         else samOf(pt)
 
       /* The SAM case comes first so that this works:
@@ -2848,15 +2868,11 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
        * Note that the arity of the sam must correspond to the arity of the function.
        */
       val samViable = sam.exists && sameLength(sam.info.params, fun.vparams)
+      val ptNorm = if (samViable) samToFunctionType(pt, sam) else pt
       val (argpts, respt) =
-        if (samViable) {
-          val samInfo = pt memberInfo sam
-          (samInfo.paramTypes, samInfo.resultType)
-        } else {
-          pt baseType FunctionSymbol match {
-            case TypeRef(_, FunctionSymbol, args :+ res) => (args, res)
-            case _                                       => (fun.vparams map (_ => if (pt == ErrorType) ErrorType else NoType), WildcardType)
-          }
+        ptNorm baseType FunctionSymbol match {
+          case TypeRef(_, FunctionSymbol, args :+ res) => (args, res)
+          case _                                       => (fun.vparams map (_ => if (pt == ErrorType) ErrorType else NoType), WildcardType)
         }
 
       if (!FunctionSymbol.exists)