SI-8040 Warn unused parameters

One can `-Ywarn-unused:params` or more narrowly warn only for
unused implicit parameters with `-Ywarn-unused:implicits`.

Params includes constructor parameters.

The settings for privates and locals are not yet distinguished.

```
$ skalac -Ywarn-unused:help
Enable or disable specific `unused' warnings
  imports    Warn if an import selector is not referenced.
  patvars    Warn if a variable bound in a pattern is unused.
  privates   Warn if a private member is unused.
  locals     Warn if a local definition is unused.
  params     Warn if a value parameter is unused.
  implicits  Warn if an implicit parameter is unused.
```

1 files changed, 32 insertions, 0 deletions

diff --git a/test/files/neg/warn-unused-implicits.scala b/test/files/neg/warn-unused-implicits.scala
new file mode 100644
index 0000000000..54f924eac0
--- /dev/null
+++ b/test/files/neg/warn-unused-implicits.scala
@@ -0,0 +1,32 @@
+
+trait InterFace {
+  /** Call something. */
+  def call(a: Int, b: String, c: Double)(implicit s: String): Int
+}
+
+trait BadAPI extends InterFace {
+  def f(a: Int,
+        b: String,
+        c: Double
+       )(implicit s: String): Int = {  // warn
+    println(b + c)
+    a
+  }
+  @deprecated ("no warn in deprecated API", since="yesterday")
+  def g(a: Int,
+        b: String,
+        c: Double
+       )(implicit s: String): Int = {  // no warn
+    println(b + c)
+    a
+  }
+  override def call(a: Int,
+                    b: String,
+                    c: Double
+                   )(implicit s: String): Int = {  // no warn, required by superclass
+    println(b + c)
+    a
+  }
+
+  def i(implicit s: String, t: Int) = t           // yes, warn
+}