SI-6947 Better type parameter names for Map classes

Type parameter names are currently assigned pretty much alphabetically
without any meaning. This change renames all key parameters in Map
classes from `A` to `K` and all value parameters from `B` to `V` to
make them more meaningful. Derived names are renamed accordingly (e.g.
`V1` instead of `B1` for an upper bound on `V`, `W` instead of `C` for
a new value type).

As a side-effect this solves the documentation problem in SI-6947. Due
to using `B` both as a type parameter for `foldLeft[B]` in
`GenTraversableOnce[A]` and in `Map[A, B]` which extends
`GenTraversableOnce[(A, B)]`, the signature of `Map.foldLeft` was
rendered in scaladoc as

    def foldLeft[B](z: B)(op: (B, (A, B)) ⇒ B): B 

Now you get an unambiguous version:

    def foldLeft[B](z: B)(op: (B, (K, V)) ⇒ B): B

Author: szeiger

src/library/scala/collection/GenMap.scala

@@ -18,18 +18,18 @@ import generic._
  *  @author Aleksandar Prokopec
  *  @since 2.9
  */
-trait GenMap[A, +B]
-extends GenMapLike[A, B, GenMap[A, B]]
-   with GenIterable[(A, B)]
+trait GenMap[K, +V]
+extends GenMapLike[K, V, GenMap[K, V]]
+   with GenIterable[(K, V)]
 {
-  def seq: Map[A, B]
+  def seq: Map[K, V]
 
-  def updated [B1 >: B](key: A, value: B1): GenMap[A, B1]
+  def updated [V1 >: V](key: K, value: V1): GenMap[K, V1]
 }
 
 object GenMap extends GenMapFactory[GenMap] {
-  def empty[A, B]: immutable.Map[A, B] = immutable.Map.empty
+  def empty[K, V]: immutable.Map[K, V] = immutable.Map.empty
 
   /** $mapCanBuildFromInfo */
-  implicit def canBuildFrom[A, B]: CanBuildFrom[Coll, (A, B), GenMap[A, B]] = new MapCanBuildFrom[A, B]
+  implicit def canBuildFrom[K, V]: CanBuildFrom[Coll, (K, V), GenMap[K, V]] = new MapCanBuildFrom[K, V]
 }

src/library/scala/collection/GenMapLike.scala

@@ -22,13 +22,13 @@ package collection
  *  A map is a collection of bindings from keys to values, where there are
  *  no duplicate keys.
  */
-trait GenMapLike[A, +B, +Repr] extends GenIterableLike[(A, B), Repr] with Equals with Parallelizable[(A, B), parallel.ParMap[A, B]] {
-  def default(key: A): B
-  def get(key: A): Option[B]
-  def apply(key: A): B
-  def seq: Map[A, B]
-  def +[B1 >: B](kv: (A, B1)): GenMap[A, B1]
-  def - (key: A): Repr
+trait GenMapLike[K, +V, +Repr] extends GenIterableLike[(K, V), Repr] with Equals with Parallelizable[(K, V), parallel.ParMap[K, V]] {
+  def default(key: K): V
+  def get(key: K): Option[V]
+  def apply(key: K): V
+  def seq: Map[K, V]
+  def +[V1 >: V](kv: (K, V1)): GenMap[K, V1]
+  def - (key: K): Repr
 
   // This hash code must be symmetric in the contents but ought not
   // collide trivially.
@@ -41,17 +41,17 @@ trait GenMapLike[A, +B, +Repr] extends GenIterableLike[(A, B), Repr] with Equals
    *   @tparam  B1       the result type of the default computation.
    *   @return  the value associated with `key` if it exists,
    *            otherwise the result of the `default` computation.
-   *   @usecase def getOrElse(key: A, default: => B): B
+   *   @usecase def getOrElse(key: K, default: => V): V
    *     @inheritdoc
    */
-  def getOrElse[B1 >: B](key: A, default: => B1): B1
+  def getOrElse[V1 >: V](key: K, default: => V1): V1
 
   /** Tests whether this map contains a binding for a key.
    *
    *  @param key the key
    *  @return    `true` if there is a binding for `key` in this map, `false` otherwise.
    */
-  def contains(key: A): Boolean
+  def contains(key: K): Boolean
 
   /** Tests whether this map contains a binding for a key. This method,
    *  which implements an abstract method of trait `PartialFunction`,
@@ -60,47 +60,47 @@ trait GenMapLike[A, +B, +Repr] extends GenIterableLike[(A, B), Repr] with Equals
    *  @param key the key
    *  @return    `true` if there is a binding for `key` in this map, `false` otherwise.
    */
-  def isDefinedAt(key: A): Boolean
+  def isDefinedAt(key: K): Boolean
 
-  def keySet: GenSet[A]
+  def keySet: GenSet[K]
 
   /** Collects all keys of this map in an iterable collection.
    *
    *  @return the keys of this map as an iterable.
    */
-  def keys: GenIterable[A]
+  def keys: GenIterable[K]
 
   /** Collects all values of this map in an iterable collection.
    *
    *  @return the values of this map as an iterable.
    */
-  def values: GenIterable[B]
+  def values: GenIterable[V]
 
   /** Creates an iterator for all keys.
    *
    *  @return an iterator over all keys.
    */
-  def keysIterator: Iterator[A]
+  def keysIterator: Iterator[K]
 
   /** Creates an iterator for all values in this map.
    *
    *  @return an iterator over all values that are associated with some key in this map.
    */
-  def valuesIterator: Iterator[B]
+  def valuesIterator: Iterator[V]
 
   /** Filters this map by retaining only keys satisfying a predicate.
    *  @param  p   the predicate used to test keys
    *  @return an immutable map consisting only of those key value pairs of this map where the key satisfies
    *          the predicate `p`. The resulting map wraps the original map without copying any elements.
    */
-  def filterKeys(p: A => Boolean): GenMap[A, B]
+  def filterKeys(p: K => Boolean): GenMap[K, V]
 
   /** Transforms this map by applying a function to every retrieved value.
    *  @param  f   the function used to transform values of this map.
    *  @return a map view which maps every key of this map
    *          to `f(this(key))`. The resulting map wraps the original map without copying any elements.
    */
-  def mapValues[C](f: B => C): GenMap[A, C]
+  def mapValues[W](f: V => W): GenMap[K, W]
 
   /** Compares two maps structurally; i.e., checks if all mappings
    *  contained in this map are also contained in the other map,

src/library/scala/collection/Map.scala

@@ -12,7 +12,7 @@ package collection
 import generic._
 
 /**
- *  A map from keys of type `A` to values of type `B`.
+ *  A map from keys of type `K` to values of type `V`.
  *
  *  $mapNote
  *
@@ -22,38 +22,38 @@ import generic._
  *  '''Note:''' If your additions and mutations return the same kind of map as the map
  *        you are defining, you should inherit from `MapLike` as well.
  *
- *  @tparam A     the type of the keys in this map.
- *  @tparam B     the type of the values associated with keys.
+ *  @tparam K     the type of the keys in this map.
+ *  @tparam V     the type of the values associated with keys.
  *
  *  @since 1.0
  */
-trait Map[A, +B] extends Iterable[(A, B)] with GenMap[A, B] with MapLike[A, B, Map[A, B]] {
-  def empty: Map[A, B] = Map.empty
+trait Map[K, +V] extends Iterable[(K, V)] with GenMap[K, V] with MapLike[K, V, Map[K, V]] {
+  def empty: Map[K, V] = Map.empty
 
-  override def seq: Map[A, B] = this
+  override def seq: Map[K, V] = this
 }
 
 /** $factoryInfo
  *  @define Coll `Map`
  *  @define coll map
  */
 object Map extends MapFactory[Map] {
-  def empty[A, B]: immutable.Map[A, B] = immutable.Map.empty
+  def empty[K, V]: immutable.Map[K, V] = immutable.Map.empty
 
   /** $mapCanBuildFromInfo */
-  implicit def canBuildFrom[A, B]: CanBuildFrom[Coll, (A, B), Map[A, B]] = new MapCanBuildFrom[A, B]
+  implicit def canBuildFrom[K, V]: CanBuildFrom[Coll, (K, V), Map[K, V]] = new MapCanBuildFrom[K, V]
 
   /** An abstract shell used by { mutable, immutable }.Map but not by collection.Map
    *  because of variance issues.
    */
-  abstract class WithDefault[A, +B](underlying: Map[A, B], d: A => B) extends AbstractMap[A, B] with Map[A, B] with Serializable {
+  abstract class WithDefault[K, +V](underlying: Map[K, V], d: K => V) extends AbstractMap[K, V] with Map[K, V] with Serializable {
     override def size               = underlying.size
-    def get(key: A)                 = underlying.get(key) // removed in 2.9: orElse Some(default(key))
+    def get(key: K)                 = underlying.get(key) // removed in 2.9: orElse Some(default(key))
     def iterator                    = underlying.iterator
-    override def default(key: A): B = d(key)
+    override def default(key: K): V = d(key)
   }
 
 }
 
 /** Explicit instantiation of the `Map` trait to reduce class file size in subclasses. */
-abstract class AbstractMap[A, +B] extends AbstractIterable[(A, B)] with Map[A, B]
+abstract class AbstractMap[K, +V] extends AbstractIterable[(K, V)] with Map[K, V]