Improved @tailrec error messages to specify the...

Improved @tailrec error messages to specify the reason. In the process
fixed old bug involving tail call transformation. Closes #3275, #2018.
Review by dragos.

3 files changed, 55 insertions, 22 deletions

diff --git a/test/files/neg/tailrec.check b/test/files/neg/tailrec.check
index 22d70e82a0..27d99f632e 100644
--- a/test/files/neg/tailrec.check
+++ b/test/files/neg/tailrec.check
@@ -1,10 +1,16 @@
-tailrec.scala:6: error: could not optimize @tailrec annotated method
+tailrec.scala:43: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
   def facfail(n: Int): Int =
       ^
-tailrec.scala:42: error: could not optimize @tailrec annotated method
+tailrec.scala:50: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
   @tailrec def fail1(x: Int): Int = fail1(x)
                ^
-tailrec.scala:45: error: could not optimize @tailrec annotated method
-  @tailrec def fail2[T](xs: List[T]): List[T] = xs match {
-               ^
-three errors found
+tailrec.scala:53: error: could not optimize @tailrec annotated method: it contains a recursive call not in tail position
+  @tailrec final def fail2[T](xs: List[T]): List[T] = xs match {
+                     ^
+tailrec.scala:59: error: could not optimize @tailrec annotated method: it is called recursively with different type arguments
+  @tailrec final def fail3[T](x: Int): Int = fail3(x - 1)
+                     ^
+tailrec.scala:63: error: could not optimize @tailrec annotated method: it changes type of 'this' on a polymorphic recursive call
+    @tailrec final def fail4[U](other: Tom[U], x: Int): Int = other.fail4[U](other, x - 1)
+                       ^
+5 errors found
diff --git a/test/files/neg/tailrec.scala b/test/files/neg/tailrec.scala
index 4c45672f93..a77f439cfe 100644
--- a/test/files/neg/tailrec.scala
+++ b/test/files/neg/tailrec.scala
@@ -1,53 +1,65 @@
 import scala.annotation.tailrec
 
 // putting @tailrec through the paces
-object Main {
-  @tailrec
-  def facfail(n: Int): Int =
-    if (n == 0) 1
-    else n * facfail(n - 1)
-
+object Winners {
   @tailrec
   def facsucc(n: Int, acc: Int): Int =
     if (n == 0) acc
     else facsucc(n - 1, n * acc)
 
-  @tailrec def loopy1(x: Int): Int = loopy1(x - 1)
+  @tailrec def loopsucc1(x: Int): Int = loopsucc1(x - 1)
+  @tailrec def loopsucc2[T](x: Int): Int = loopsucc2[T](x - 1)
 
   def ding {
     object dong {
-      @tailrec def loopy2(x: Int): Int = loopy2(x)
+      @tailrec def loopsucc3(x: Int): Int = loopsucc3(x)
     }
     ()
   }
 
   def inner(q: Int) = {
     @tailrec
-    def loopy3(x: Int): Int = loopy3(x + 1)
+    def loopsucc4(x: Int): Int = loopsucc4(x + 1)
 
-    loopy3(q)
+    loopsucc4(q)
+  }
+
+  object innerBob {
+    @tailrec def loopsucc5(x: Int): Int = loopsucc5(x)
   }
 }
 
-class Bob {
-  // these should work
+class Winners {
   @tailrec private def succ1(x: Int): Int = succ1(x)
   @tailrec final def succ2(x: Int): Int = succ2(x)
   @tailrec final def succ3[T](in: List[T], acc: List[T]): List[T] = in match {
     case Nil      => Nil
     case x :: xs  => succ3(xs, x :: acc)
   }
+}
 
+object Failures {
+  @tailrec
+  def facfail(n: Int): Int =
+    if (n == 0) 1
+    else n * facfail(n - 1)
+}
+
+class Failures {
   // not private, not final
   @tailrec def fail1(x: Int): Int = fail1(x)
 
   // a typical between-chair-and-keyboard error
-  @tailrec def fail2[T](xs: List[T]): List[T] = xs match {
+  @tailrec final def fail2[T](xs: List[T]): List[T] = xs match {
     case Nil      => Nil
-    case x :: xs  => x :: fail2(xs)
+    case x :: xs  => x :: fail2[T](xs)
   }
 
-  object innerBob {
-    @tailrec def succ4(x: Int): Int = succ4(x)
+  // unsafe
+  @tailrec final def fail3[T](x: Int): Int = fail3(x - 1)
+
+  // unsafe
+  class Tom[T](x: Int) {
+    @tailrec final def fail4[U](other: Tom[U], x: Int): Int = other.fail4[U](other, x - 1)
   }
 }
diff --git a/test/files/pos/bug2018.scala b/test/files/pos/bug2018.scala
new file mode 100644
index 0000000000..1736c394c9
--- /dev/null
+++ b/test/files/pos/bug2018.scala
@@ -0,0 +1,15 @@
+class A {
+  val b = new B
+
+  def getChildren = List(new A).iterator
+
+  class B {
+    private def check = true
+
+    private def getAncestor(p: A): A = {
+      val c = (p.getChildren.find(_.b.check)) match {case Some(d) => d case None => p}
+
+      if (c == p) p else c.b.getAncestor(c)
+    }
+  }
+}
\ No newline at end of file