SD-121 Remove now-unneeded J{Function,Proc}N functional interfaces

Non specialized functions can directly use `scala.FunctionN` as the
functional interface, now that mixin generates default methods in
the new trait encoding.

Unfortunately we can't do this for specialized functions as things
stand: specialization leaves the wrong method abstract. In principle,
we could/should amend the specialization transform to fix this. But
my earlier attempts at this weren't sucessful, so for now we have
to stick with the fallback plan of keeping some hand rolled functional
interfaces around.

This commit reduces the surface area of `scala.runtime.java8` to
the minimal requiremnt: one functional interface for each specialized
variant of `Function{0,1,2}`.

1 files changed, 6 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/Delambdafy.scala b/src/compiler/scala/tools/nsc/transform/Delambdafy.scala
index 804bcddb7b..88837842fa 100644
--- a/src/compiler/scala/tools/nsc/transform/Delambdafy.scala
+++ b/src/compiler/scala/tools/nsc/transform/Delambdafy.scala
@@ -241,8 +241,12 @@ abstract class Delambdafy extends Transform with TypingTransformers with ast.Tre
               exitingErasure(target.info.paramTypes).map(reboxValueClass) :+ reboxValueClass(exitingErasure(target.info.resultType))).toTypeName
 
           val isSpecialized = specializedName != funSym.name
-          val functionalInterface = // TODO: this is no longer needed, right? we can just use the regular function classes
-            if (isSpecialized) currentRun.runDefinitions.Scala_Java8_CompatPackage.info.decl(specializedName.prepend("J"))
+          val functionalInterface =
+            if (isSpecialized) {
+              // Unfortunately we still need to use custom functional interfaces for specialized functions so that the
+              // unboxed apply method is left abstract for us to implement.
+              currentRun.runDefinitions.Scala_Java8_CompatPackage.info.decl(specializedName.prepend("J"))
+            }
             else FunctionClass(originalFunction.vparams.length)
 
           (functionalInterface, isSpecialized)