Disable -Ydelambdafy:method for specialized FunctionN

The Delambdafy phase generates its `FunctionN` subclasses after
the specialization phase. As such, `((x: Int) => x).apply(42)` incurs
boxing.

This commit falls back to the `-Ydelambdafy:inline` in this case.

This is done by running the specialization type map over the
type of the function, and seeing if anything changes. To make this
work robustly, we first need to ensure that the specialization info
transformer has processed all the function types.

This is not a fundamental limitation; we could in principle generate
the specialized code.

A followup change will use `-Ydelambdafy:method` as the basis for
invokedymnamic lambdas. As part of that stream of
work, we will synthesize specialization-aware lambdas, and remove
the fallback to `-Ydelambdafy:inline`.

I have updated some tests that intend to test the delambdafy transform
to avoid use of specialized function types.

1 files changed, 17 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/UnCurry.scala b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
index 3544dc9966..001fd71bfb 100644
--- a/src/compiler/scala/tools/nsc/transform/UnCurry.scala
+++ b/src/compiler/scala/tools/nsc/transform/UnCurry.scala
@@ -70,6 +70,14 @@ abstract class UnCurry extends InfoTransform
     private val noApply           = mutable.HashSet[Tree]()
     private val newMembers        = mutable.Map[Symbol, mutable.Buffer[Tree]]()
 
+    private lazy val forceSpecializationInfoTransformOfFunctionN: Unit = {
+      if (currentRun.specializePhase != NoPhase) { // be robust in case of -Ystop-after:uncurry
+        exitingSpecialize {
+          FunctionClass.seq.foreach(cls => cls.info)
+        }
+      }
+    }
+
     /** Add a new synthetic member for `currentOwner` */
     private def addNewMember(t: Tree): Unit =
       newMembers.getOrElseUpdate(currentOwner, mutable.Buffer()) += t
@@ -220,8 +228,16 @@ abstract class UnCurry extends InfoTransform
           def mkMethod(owner: Symbol, name: TermName, additionalFlags: FlagSet = NoFlags): DefDef =
             gen.mkMethodFromFunction(localTyper)(fun, owner, name, additionalFlags)
 
-          val canUseDelamdafyMethod = (inConstructorFlag == 0) // Avoiding synthesizing code prone to SI-6666, SI-8363 by using old-style lambda translation
+          def isSpecialized = {
+            forceSpecializationInfoTransformOfFunctionN
+            val specialized = specializeTypes.specializedType(fun.tpe)
+            !(specialized =:= fun.tpe)
+          }
 
+          def canUseDelamdafyMethod = (
+               (inConstructorFlag == 0) // Avoiding synthesizing code prone to SI-6666, SI-8363 by using old-style lambda translation
+            && !isSpecialized           // DelambdafyTransformer currently only emits generic FunctionN-s, use the old style in the meantime
+          )
           if (inlineFunctionExpansion || !canUseDelamdafyMethod) {
             val parents = addSerializable(abstractFunctionForFunctionType(fun.tpe))
             val anonClass = fun.symbol.owner newAnonymousFunctionClass(fun.pos, inConstructorFlag) addAnnotation SerialVersionUIDAnnotation