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.
```

10 files changed, 132 insertions, 4 deletions

diff --git a/test/files/neg/warn-unused-implicits.check b/test/files/neg/warn-unused-implicits.check
new file mode 100644
index 0000000000..4cc5836800
--- /dev/null
+++ b/test/files/neg/warn-unused-implicits.check
@@ -0,0 +1,9 @@
+warn-unused-implicits.scala:11: warning: parameter value s in method f is never used
+       )(implicit s: String): Int = {  // warn
+                  ^
+warn-unused-implicits.scala:31: warning: parameter value s in method i is never used
+  def i(implicit s: String, t: Int) = t           // yes, warn
+                 ^
+error: No warnings can be incurred under -Xfatal-warnings.
+two warnings found
+one error found
diff --git a/test/files/neg/warn-unused-implicits.flags b/test/files/neg/warn-unused-implicits.flags
new file mode 100644
index 0000000000..18169f3218
--- /dev/null
+++ b/test/files/neg/warn-unused-implicits.flags
@@ -0,0 +1 @@
+-Ywarn-unused:implicits -Xfatal-warnings
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
+}
diff --git a/test/files/neg/warn-unused-params.check b/test/files/neg/warn-unused-params.check
new file mode 100644
index 0000000000..ca6320ccd9
--- /dev/null
+++ b/test/files/neg/warn-unused-params.check
@@ -0,0 +1,18 @@
+warn-unused-params.scala:9: warning: parameter value b in method f is never used
+        b: String,               // warn
+        ^
+warn-unused-params.scala:32: warning: parameter value s in method i is never used
+  def i(implicit s: String) = 42           // yes, warn
+                 ^
+warn-unused-params.scala:49: warning: parameter value u in class Unusing is never used
+class Unusing(u: Int) {       // warn
+              ^
+warn-unused-params.scala:57: warning: parameter value s in class CaseyAtTheBat is never used
+case class CaseyAtTheBat(k: Int)(s: String)        // warn
+                                 ^
+warn-unused-params.scala:60: warning: parameter value readResolve in method f is never used
+  def f(readResolve: Int) = 42           // warn
+        ^
+error: No warnings can be incurred under -Xfatal-warnings.
+5 warnings found
+one error found
diff --git a/test/files/neg/warn-unused-params.flags b/test/files/neg/warn-unused-params.flags
new file mode 100644
index 0000000000..795fb74272
--- /dev/null
+++ b/test/files/neg/warn-unused-params.flags
@@ -0,0 +1 @@
+-Ywarn-unused:params -Xfatal-warnings
diff --git a/test/files/neg/warn-unused-params.scala b/test/files/neg/warn-unused-params.scala
new file mode 100644
index 0000000000..c7578e53a4
--- /dev/null
+++ b/test/files/neg/warn-unused-params.scala
@@ -0,0 +1,61 @@
+
+trait InterFace {
+  /** Call something. */
+  def call(a: Int, b: String, c: Double): Int
+}
+
+trait BadAPI extends InterFace {
+  def f(a: Int,
+        b: String,               // warn
+        c: Double): Int = {
+    println(c)
+    a
+  }
+  @deprecated ("no warn in deprecated API", since="yesterday")
+  def g(a: Int,
+        b: String,               // no warn
+        c: Double): Int = {
+    println(c)
+    a
+  }
+  override def call(a: Int,
+                    b: String,               // no warn, required by superclass
+                    c: Double): Int = {
+    println(c)
+    a
+  }
+
+  def meth(x: Int) = x
+
+  override def equals(other: Any): Boolean = true  // no warn
+
+  def i(implicit s: String) = 42           // yes, warn
+
+  /*
+  def future(x: Int): Int = {
+    val y = 42
+    val x = y               // maybe option to warn only if shadowed
+    x
+  }
+  */
+}
+
+// mustn't alter warnings in super
+trait PoorClient extends BadAPI {
+  override def meth(x: Int) = ???       // no warn
+  override def f(a: Int, b: String, c: Double): Int = a + b.toInt + c.toInt
+}
+
+class Unusing(u: Int) {       // warn
+  def f = ???
+}
+
+class Valuing(val u: Int)        // no warn
+
+case class CaseyKasem(k: Int)        // no warn
+
+case class CaseyAtTheBat(k: Int)(s: String)        // warn
+
+trait Ignorance {
+  def f(readResolve: Int) = 42           // warn
+}
diff --git a/test/files/neg/warn-unused-patvars.check b/test/files/neg/warn-unused-patvars.check
index 002f7a07ca..2665126a36 100644
--- a/test/files/neg/warn-unused-patvars.check
+++ b/test/files/neg/warn-unused-patvars.check
@@ -1,10 +1,10 @@
-warn-unused-patvars.scala:7: warning: private val x in trait Boundings is never used
+warn-unused-patvars.scala:9: warning: private val x in trait Boundings is never used
   private val x = 42                      // warn, sanity check
               ^
-warn-unused-patvars.scala:26: warning: local val x in method v is never used
+warn-unused-patvars.scala:28: warning: local val x in method v is never used
     val D(x) = d                          // warn, fixme
          ^
-warn-unused-patvars.scala:30: warning: local val x in method w is never used
+warn-unused-patvars.scala:32: warning: local val x in method w is never used
     val D(x @ _) = d                      // warn, fixme (valdef pos is different)
          ^
 error: No warnings can be incurred under -Xfatal-warnings.
diff --git a/test/files/neg/warn-unused-patvars.scala b/test/files/neg/warn-unused-patvars.scala
index 6f4620c0c7..3d35dfedd6 100644
--- a/test/files/neg/warn-unused-patvars.scala
+++ b/test/files/neg/warn-unused-patvars.scala
@@ -1,4 +1,6 @@
 
+// verify no warning when -Ywarn-unused:-patvars
+
 case class C(a: Int, b: String, c: Option[String])
 case class D(a: Int)
 
diff --git a/test/files/neg/warn-unused-privates.flags b/test/files/neg/warn-unused-privates.flags
index 25474aefb3..5ab4f36371 100644
--- a/test/files/neg/warn-unused-privates.flags
+++ b/test/files/neg/warn-unused-privates.flags
@@ -1 +1 @@
--Ywarn-unused -Xfatal-warnings
+-Ywarn-unused:privates,locals,patvars -Xfatal-warnings
diff --git a/test/files/neg/warn-unused-privates.scala b/test/files/neg/warn-unused-privates.scala
index bc2799067e..f0580f02d5 100644
--- a/test/files/neg/warn-unused-privates.scala
+++ b/test/files/neg/warn-unused-privates.scala
@@ -189,3 +189,7 @@ trait Forever {
     } yield 42                           // val emitted only if needed, hence nothing unused
   }
 }
+
+trait Ignorance {
+  private val readResolve = 42      // ignore
+}