SI-8359 Limit change in param order to delamdafy targets

The previous commit described three options for making our
lambda targets amenable to use in LambdaMetafactory. It implemented
a broad change (Option #1), which changed the parameter order of
all lambda-lifted methods.

This commit pares things back to only make this change for
synthetic methods that hold lambda bodies. This is a more
conservative approach for the 2.11.x series.

Author: retronym

src/compiler/scala/tools/nsc/transform/LambdaLift.scala

@@ -563,7 +563,10 @@ abstract class LambdaLift extends InfoTransform {
   } // class LambdaLifter
 
   private def addFree[A](sym: Symbol, free: List[A], original: List[A]): List[A] = {
-    val prependFree = !sym.isConstructor
+    val prependFree = (
+         !sym.isConstructor // this condition is redundant for now. It will be needed if we remove the second condition in 2.12.x
+      && (settings.Ydelambdafy.value == "method" && sym.isDelambdafyTarget) // SI-8359 Makes the lambda body a viable as the target MethodHandle for a call to LambdaMetafactory
+    )
     if (prependFree) free ::: original
     else             original ::: free
   }

src/reflect/scala/reflect/internal/Symbols.scala

@@ -789,6 +789,7 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
 
     final def isAnonymousFunction = isSynthetic && (name containsName tpnme.ANON_FUN_NAME)
     final def isDelambdafyFunction = isSynthetic && (name containsName tpnme.DELAMBDAFY_LAMBDA_CLASS_NAME)
+    final def isDelambdafyTarget  = isSynthetic && isMethod && (name containsName tpnme.ANON_FUN_NAME)
     final def isDefinedInPackage  = effectiveOwner.isPackageClass
     final def needsFlatClasses    = phase.flatClasses && rawowner != NoSymbol && !rawowner.isPackageClass