Merge pull request scala#2717 from retronym/ticket/6574

SI-6574 Support @tailrec for extension methods.

Author: adriaanm

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

@@ -208,6 +208,7 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
               companion.moduleClass.newMethod(extensionName, origMeth.pos, origMeth.flags & ~OVERRIDE & ~PROTECTED | FINAL)
                 setAnnotations origMeth.annotations
             )
+            origMeth.removeAnnotation(TailrecClass) // it's on the extension method, now.
             companion.info.decls.enter(extensionMeth)
           }
 
@@ -221,15 +222,16 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
           val extensionParams = allParameters(extensionMono)
           val extensionThis   = gen.mkAttributedStableRef(thiz setPos extensionMeth.pos)
 
-          val extensionBody = (
-            rhs
+          val extensionBody: Tree = {
+            val tree = rhs
               .substituteSymbols(origTpeParams, extensionTpeParams)
               .substituteSymbols(origParams, extensionParams)
               .substituteThis(origThis, extensionThis)
               .changeOwner(origMeth -> extensionMeth)
-          )
+            new SubstututeRecursion(origMeth, extensionMeth, unit).transform(tree)
+          }
 
-          // Record the extension method ( FIXME: because... ? )
+          // Record the extension method. Later, in `Extender#transformStats`, these will be added to the companion object.
           extensionDefs(companion) += atPos(tree.pos)(DefDef(extensionMeth, extensionBody))
 
           // These three lines are assembling Foo.bar$extension[T1, T2, ...]($this)
@@ -264,4 +266,33 @@ abstract class ExtensionMethods extends Transform with TypingTransformers {
           stat
       }
   }
+
+  final class SubstututeRecursion(origMeth: Symbol, extensionMeth: Symbol,
+                            unit: CompilationUnit) extends TypingTransformer(unit) {
+    override def transform(tree: Tree): Tree = tree match {
+      // SI-6574 Rewrite recursive calls against the extension method so they can
+      //         be tail call optimized later. The tailcalls phases comes before
+      //         erasure, which performs this translation more generally at all call
+      //         sites.
+      //
+      //         // Source
+      //         class C[C] { def meth[M](a: A) = { { <expr>: C[C'] }.meth[M'] } }
+      //
+      //         // Translation
+      //         class C[C] { def meth[M](a: A) = { { <expr>: C[C'] }.meth[M'](a1) } }
+      //         object C   { def meth$extension[M, C](this$: C[C], a: A)
+      //                        = { meth$extension[M', C']({ <expr>: C[C'] })(a1) } }
+      case treeInfo.Applied(sel @ Select(qual, _), targs, argss) if sel.symbol == origMeth =>
+        import gen.CODE._
+        localTyper.typedPos(tree.pos) {
+          val allArgss = List(qual) :: argss
+          val origThis = extensionMeth.owner.companionClass
+          val baseType = qual.tpe.baseType(origThis)
+          val allTargs = targs.map(_.tpe) ::: baseType.typeArgs
+          val fun = gen.mkAttributedTypeApply(THIS(extensionMeth.owner), extensionMeth, allTargs)
+          allArgss.foldLeft(fun)(Apply(_, _))
+        }
+      case _ => super.transform(tree)
+    }
+  }
 }

test/files/neg/t6574.check

@@ -0,0 +1,7 @@
+t6574.scala:4: error: could not optimize @tailrec annotated method notTailPos$extension: it contains a recursive call not in tail position
+    println("tail")
+    ^
+t6574.scala:8: error: could not optimize @tailrec annotated method differentTypeArgs$extension: it is called recursively with different type arguments
+    {(); new Bad[String, Unit](0)}.differentTypeArgs
+                                   ^
+two errors found

test/files/neg/t6574.scala

@@ -0,0 +1,10 @@
+class Bad[X, Y](val v: Int) extends AnyVal {
+  @annotation.tailrec final def notTailPos[Z](a: Int)(b: String) {
+    this.notTailPos[Z](a)(b)
+    println("tail")
+  }
+
+  @annotation.tailrec final def differentTypeArgs {
+    {(); new Bad[String, Unit](0)}.differentTypeArgs
+  }
+}

test/files/pos/t6574.scala

@@ -0,0 +1,19 @@
+class Bad[X, Y](val v: Int) extends AnyVal {
+  def vv = v
+  @annotation.tailrec final def foo[Z](a: Int)(b: String) {
+    this.foo[Z](a)(b)
+  }
+
+  @annotation.tailrec final def differentReceiver {
+    {(); new Bad[X, Y](0)}.differentReceiver
+  }
+
+  @annotation.tailrec final def dependent[Z](a: Int)(b: String): b.type = {
+    this.dependent[Z](a)(b)
+  }
+}
+
+class HK[M[_]](val v: Int) extends AnyVal {
+  def hk[N[_]]: Unit = if (false) hk[M] else ()
+}
+

test/files/run/t6574b.check

@@ -0,0 +1 @@
+List(5, 4, 3, 2, 1)

test/files/run/t6574b.scala

@@ -0,0 +1,7 @@
+object Test extends App {
+  implicit class AnyOps(val i: Int) extends AnyVal {
+    private def parentsOf(x: Int): List[Int] = if (x == 0) Nil else x :: parentsOf(x - 1)
+    def parents: List[Int] = parentsOf(i)
+  }
+  println((5).parents)
+}