A pretty severe bug in the recognition of tail ...

A pretty severe bug in the recognition of tail call elimination.
It turns out that Tailcalls will perform "partial elimination" in
situations such as:

  @annotation.tailrec final def f(x: Int): Int = f(f(x))

The outer call to f1 becomes a jump, but the inner call remains as
it was. I implemented @tailrec under the impression that if the
optimization had taken place, it had gone all the way. So this is now
fixed with a direct examination of the rewritten tree.

While I was in there I threw in some improved error reporting: the error
positioning is now on the call which is not in tail position rather than
the method declaration.

Closes #4135, no review.

4 files changed, 38 insertions, 14 deletions

diff --git a/test/files/neg/tailrec-2.check b/test/files/neg/tailrec-2.check
index ab6733946d..4f763a1c8d 100644
--- a/test/files/neg/tailrec-2.check
+++ b/test/files/neg/tailrec-2.check
@@ -1,4 +1,4 @@
-tailrec-2.scala:6: error: could not optimize @tailrec annotated method: it contains a recursive call targetting a supertype
+tailrec-2.scala:6: error: could not optimize @tailrec annotated method f: it contains a recursive call targetting a supertype
   @annotation.tailrec final def f[B >: A](mem: List[B]): List[B] = (null: Super[A]).f(mem)
-                                ^
+                                                                                    ^
 one error found
diff --git a/test/files/neg/tailrec-3.check b/test/files/neg/tailrec-3.check
new file mode 100644
index 0000000000..a3542fb564
--- /dev/null
+++ b/test/files/neg/tailrec-3.check
@@ -0,0 +1,10 @@
+tailrec-3.scala:4: error: could not optimize @tailrec annotated method quux: it contains a recursive call not in tail position
+  @tailrec private def quux(xs: List[String]): List[String] = quux(quux(xs))
+                                                                       ^
+tailrec-3.scala:6: error: could not optimize @tailrec annotated method quux2: it contains a recursive call not in tail position
+    case x1 :: x2 :: rest => quux2(x1 :: quux2(rest))
+                                              ^
+tailrec-3.scala:10: error: could not optimize @tailrec annotated method quux3: it contains a recursive call not in tail position
+    case x :: xs if quux3(List("abc"))  => quux3(xs)
+                         ^
+three errors found
diff --git a/test/files/neg/tailrec-3.scala b/test/files/neg/tailrec-3.scala
new file mode 100644
index 0000000000..20361658ea
--- /dev/null
+++ b/test/files/neg/tailrec-3.scala
@@ -0,0 +1,14 @@
+import annotation.tailrec
+
+object Test {
+  @tailrec private def quux(xs: List[String]): List[String] = quux(quux(xs))
+  @tailrec private def quux2(xs: List[String]): List[String] = xs match {
+    case x1 :: x2 :: rest => quux2(x1 :: quux2(rest))
+    case _                => Nil
+  }
+  @tailrec private def quux3(xs: List[String]): Boolean = xs match {
+    case x :: xs if quux3(List("abc"))  => quux3(xs)
+    case _                              => false
+  }
+}
+
diff --git a/test/files/neg/tailrec.check b/test/files/neg/tailrec.check
index 27d99f632e..f6865b2f9d 100644
--- a/test/files/neg/tailrec.check
+++ b/test/files/neg/tailrec.check
@@ -1,16 +1,16 @@
-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:50: error: could not optimize @tailrec annotated method: it is neither private nor final so can be overridden
+tailrec.scala:45: error: could not optimize @tailrec annotated method facfail: it contains a recursive call not in tail position
+    else n * facfail(n - 1)
+           ^
+tailrec.scala:50: error: could not optimize @tailrec annotated method fail1: it is neither private nor final so can be overridden
   @tailrec def fail1(x: Int): Int = fail1(x)
-               ^
-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.scala:55: error: could not optimize @tailrec annotated method fail2: it contains a recursive call not in tail position
+    case x :: xs  => x :: fail2[T](xs)
+                       ^
+tailrec.scala:59: error: could not optimize @tailrec annotated method fail3: 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.scala:63: error: could not optimize @tailrec annotated method fail4: 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