Fix #5163: Properly report tailrec failures on recursive call targeting a supertype

Author: allanrenucci

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -237,8 +237,9 @@ class TailRec extends MiniPhase {
           }
           else fail("it is not in tail position")
         } else {
-          // FIXME `(method.name eq call)` is always false (Name vs Symbol). What is this trying to do?
-          val receiverIsSuper = (method.name eq call) && enclosingClass.appliedRef.widen <:< prefix.tpe.widenDealias
+          val receiverIsSuper = (method.name eq call.name) &&
+            method.matches(call) &&
+            enclosingClass.appliedRef.widen <:< prefix.tpe.widenDealias
 
           if (receiverIsSuper) fail("it contains a recursive call targeting a supertype")
           else continue

tests/neg-tailcall/t1672b.check

@@ -1,5 +1,7 @@
+import annotation.tailrec
+
 object Test1772B {
-  @annotation.tailrec
+  @tailrec
   def bar : Nothing = { // error: TailRec optimisation not applicable
     try {
       throw new RuntimeException
@@ -10,7 +12,7 @@ object Test1772B {
     }
   }
 
-  @annotation.tailrec
+  @tailrec
   def baz : Nothing = { // error: TailRec optimisation not applicable
     try {
       throw new RuntimeException
@@ -21,7 +23,7 @@ object Test1772B {
     }
   }
 
-  @annotation.tailrec
+  @tailrec
   def boz : Nothing = { // error: TailRec optimisation not applicable
     try {
       throw new RuntimeException
@@ -30,7 +32,7 @@ object Test1772B {
     }
   }
 
-  @annotation.tailrec
+  @tailrec
   def bez : Nothing = { // error: TailRec optimisation not applicable
     try {
       bez // error: it is not in tail position
@@ -40,7 +42,7 @@ object Test1772B {
   }
 
   // the `liftedTree` local method will prevent a tail call here.
-  @annotation.tailrec
+  @tailrec
   def bar(i : Int) : Int = { // error: TailRec optimisation not applicable
     if (i == 0) 0
     else 1 + (try {

tests/neg-tailcall/t1672b.scala

@@ -1,3 +1,3 @@
 object Test {
-  @annotation.tailrec def foo() = 5                   // error
+  @annotation.tailrec def foo() = 5 // error: not recursive
 }

tests/neg-tailcall/t3275.check

@@ -1,10 +1,12 @@
+import annotation.tailrec
+
 class Bad[X, Y](val v: Int) extends AnyVal {
-  @annotation.tailrec final def notTailPos[Z](a: Int)(b: String): Unit = {   // error
-    this.notTailPos[Z](a)(b)          // error
+  @tailrec final def notTailPos[Z](a: Int)(b: String): Unit = { // error: TailRec optimisation not applicable
+    this.notTailPos[Z](a)(b) // error: it is not in tail position
     println("tail")
   }
 
-  @annotation.tailrec final def differentTypeArgs: Unit = {
+  @tailrec final def differentTypeArgs: Unit = {
     {(); new Bad[String, Unit](0)}.differentTypeArgs
   }
 }

tests/neg-tailcall/t3275.scala

@@ -1,22 +1,26 @@
+import annotation.tailrec
+
 sealed abstract class Super[+A] {
   def f[B >: A](mem: List[B]) : List[B]
   def g(mem: List[_]) = ???
 }
 // This one should fail, target is a supertype
 class Bop1[+A](val element: A) extends Super[A] {
 
-  @annotation.tailrec final def f[B >: A](mem: List[B]): List[B] = (null: Super[A]).f(mem)
-  @annotation.tailrec final def f1[B >: A](mem: List[B]): List[B] = this.g(mem)
+  @tailrec final def f[B >: A](mem: List[B]): List[B] = // error: TailRec optimisation not applicable
+    (null: Super[A]).f(mem) // error: recursive call targeting a supertype
+
+  @tailrec final def f1[B >: A](mem: List[B]): List[B] = this.g(mem) // error: TailRec optimisation not applicable
 }
 // These succeed
 class Bop2[+A](val element: A) extends Super[A] {
-  @annotation.tailrec final def f[B >: A](mem: List[B]): List[B] = (null: Bop2[A]).f(mem)
+  @tailrec final def f[B >: A](mem: List[B]): List[B] = (null: Bop2[A]).f(mem)
 }
 object Bop3 extends Super[Nothing] {
-  @annotation.tailrec final def f[B](mem: List[B]): List[B] = (???: Bop3.type).f(mem) // error // error
+  @tailrec final def f[B](mem: List[B]): List[B] = (??? : Bop3.type).f(mem)
 }
 class Bop4[+A](val element: A) extends Super[A] {
-  @annotation.tailrec final def f[B >: A](mem: List[B]): List[B] = Other.f[A].f(mem)
+  @tailrec final def f[B >: A](mem: List[B]): List[B] = Other.f[A].f(mem)
 }
 
 object Other {

tests/neg-tailcall/t6574.check

@@ -1,14 +1,19 @@
 import annotation.tailrec
 
 object Test {
-  @tailrec private def quux(xs: List[String]): List[String] = quux(quux(xs)) // error
+  @tailrec private def quux(xs: List[String]): List[String] =
+    quux(
+      quux(xs) // error: not in tail position
+    )
   @tailrec private def quux2(xs: List[String]): List[String] = xs match {
-    case x1 :: x2 :: rest => quux2(x1 :: quux2(rest))                        // error
-    case _                => Nil
+    case x1 :: x2 :: rest => quux2(
+      x1 :: quux2(rest)) // error: not in tail position
+    case _ => Nil
   }
   @tailrec private def quux3(xs: List[String]): Boolean = xs match {
-    case x :: xs if quux3(List("abc"))  => quux3(xs) // error
-    case _                              => false
+    case x :: xs if quux3(List("abc")) => // error: not in tail position
+      quux3(xs)
+    case _ => false
   }
 }
 

tests/neg-tailcall/t6574.scala

@@ -0,0 +1,11 @@
+import annotation.tailrec
+
+class UnrolledBuffer {
+  def remove(idx: Int): Unit = ()
+
+  @tailrec final def remove(idx: Int, count: Int): Unit =
+    if (count > 0) {
+      remove(idx)
+      remove(idx, count - 1)
+    }
+}