Tailcalls reports less useful error

[som-snytt]

As reported on SO:

$ scala
Welcome to Scala 2.12.2 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_111).
Type in expressions for evaluation. Or try :help.

scala> import scala.annotation.tailrec
import scala.annotation.tailrec

scala> @tailrec private def loop[V](key: String): V = key match { case _ => loop[V](key) }

scala> @tailrec private def loop[V](key: String): V = key match { case _ => loop(key) }
<console>:12: error: could not optimize @tailrec annotated method loop: it contains a recursive call not in tail position
       @tailrec private def loop[V](key: String): V = key match { case _ => loop(key) }
                                                                                ^

scala> @tailrec private def loop[V](key: String): V = loop(key)
<console>:12: error: could not optimize @tailrec annotated method loop: it is called recursively with different type arguments
       @tailrec private def loop[V](key: String): V = loop(key)
                                                      ^

You'd expect the last error message also for the first message.

With -Ydebug -Ylog:tailcalls:

[log tailcalls] Cannot rewrite recursive call at: source-<console>,line-12,offset=173 because: it is called recursively with different type arguments
[log tailcalls] Cannot rewrite recursive call at: source-<console>,line-12,offset=177 because: it contains a recursive call not in tail position
<console>:12: error: could not optimize @tailrec annotated method loop: it contains a recursive call not in tail position
       @tailrec private def loop[V](key: String): V = key match { case _ => loop(key) }
                                                                                ^

On 2.13, note that with lots of debug output, we lose the source code to scrolling:

[log tailcalls] Cannot rewrite recursive call at: source-<console>,line-1,offset=69 because: it is called recursively with different type arguments
[log tailcalls] Cannot rewrite recursive call at: source-<console>,line-1,offset=73 because: it contains a recursive call not in tail position
                                                                                ^
       error: could not optimize @tailrec annotated method loop: it contains a recursive call not in tail position

[som-snytt]

presumably scala/scala#6065