Fix scala#5163: Report tailrec failures on recursive call targeting supertype

Author: allanrenucci

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

@@ -272,8 +272,8 @@ class TailRec extends MiniPhase {
         }
 
         val isRecursiveCall = call eq method
-        // FIXME `(method.name eq sym)` is always false (Name vs Symbol). What is this trying to do?
-        def isRecursiveSuperCall = (method.name eq call) &&
+        def isRecursiveSuperCall = (method.name eq call.name) &&
+          method.matches(call) &&
           enclosingClass.appliedRef.widen <:< prefix.tpe.widenDealias
 
         if (isRecursiveCall) {

tests/neg-tailcall/tailrec-2.scala

@@ -8,7 +8,7 @@ sealed abstract class Super[+A] {
 class Bop1[+A](val element: A) extends Super[A] {
 
   @tailrec final def f[B >: A](mem: List[B]): List[B] = // error: TailRec optimisation not applicable
-    (null: Super[A]).f(mem)
+    (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
 }

tests/pos/tailcall/i5163.scala

@@ -0,0 +1,9 @@
+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)
+    }
+}