Move ConfigMergeable.toValue to be private, as MergeableValue.toFallbackValue

Avoids confusion because ConfigValue.toValue was pointless and
Config.toValue was not as clear as Config.root (plus having two ways
to do it on Config was strange).

This change does require a cast in the withFallback implementation but
we already have the analogous pattern all over the place (i.e. assuming that
instances of the public interfaces are always instances of our private
implementations).

config/src/main/java/com/typesafe/config/Config.java

@@ -87,9 +87,6 @@ public interface Config extends ConfigMergeable {
     @Override
     Config withFallback(ConfigMergeable other);
 
-    @Override
-    ConfigObject toValue();
-
     /**
      * Returns a replacement config with all substitutions (the
      * <code>${foo.bar}</code> syntax, see <a

config/src/main/java/com/typesafe/config/ConfigMergeable.java

@@ -17,15 +17,6 @@ package com.typesafe.config;
  * implementations will break.
  */
 public interface ConfigMergeable {
-    /**
-     * Converts this instance to a {@link ConfigValue}. If called on a
-     * {@code ConfigValue} it returns {@code this}, if called on a
-     * {@link Config} it's equivalent to {@link Config#root()}.
-     *
-     * @return this instance as a {@code ConfigValue}
-     */
-    ConfigValue toValue();
-
     /**
      * Returns a new value computed by merging this value with another, with
      * keys in this value "winning" over the other one. Only

config/src/main/java/com/typesafe/config/impl/AbstractConfigObject.java

@@ -35,6 +35,11 @@ abstract class AbstractConfigObject extends AbstractConfigValue implements
         return config;
     }
 
+    @Override
+    public AbstractConfigObject toFallbackValue() {
+        return this;
+    }
+
     /**
      * This looks up the key with no transformation or type conversion of any
      * kind, and returns null if the key is not present.

config/src/main/java/com/typesafe/config/impl/AbstractConfigValue.java

@@ -16,7 +16,7 @@ import com.typesafe.config.ConfigValue;
  * improperly-factored and non-modular code. Please don't add parent().
  *
  */
-abstract class AbstractConfigValue implements ConfigValue {
+abstract class AbstractConfigValue implements ConfigValue, MergeableValue {
 
     final private ConfigOrigin origin;
 
@@ -72,7 +72,7 @@ abstract class AbstractConfigValue implements ConfigValue {
     }
 
     @Override
-    public AbstractConfigValue toValue() {
+    public AbstractConfigValue toFallbackValue() {
         return this;
     }
 
@@ -110,7 +110,7 @@ abstract class AbstractConfigValue implements ConfigValue {
         if (ignoresFallbacks()) {
             return this;
         } else {
-            ConfigValue other = mergeable.toValue();
+            ConfigValue other = ((MergeableValue) mergeable).toFallbackValue();
 
             if (other instanceof Unmergeable) {
                 return mergedWithTheUnmergeable((Unmergeable) other);

config/src/main/java/com/typesafe/config/impl/MergeableValue.java

@@ -0,0 +1,9 @@
+package com.typesafe.config.impl;
+
+import com.typesafe.config.ConfigMergeable;
+import com.typesafe.config.ConfigValue;
+
+interface MergeableValue extends ConfigMergeable {
+    // converts a Config to its root object and a ConfigValue to itself
+    ConfigValue toFallbackValue();
+}

config/src/main/java/com/typesafe/config/impl/SimpleConfig.java

@@ -27,7 +27,7 @@ import com.typesafe.config.ConfigValueType;
  * key-value pairs would be all the tree's leaf values, in a big flat list with
  * their full paths.
  */
-final class SimpleConfig implements Config {
+final class SimpleConfig implements Config, MergeableValue {
 
     final private AbstractConfigObject object;
 
@@ -378,7 +378,7 @@ final class SimpleConfig implements Config {
     }
 
     @Override
-    public AbstractConfigObject toValue() {
+    public AbstractConfigObject toFallbackValue() {
         return object;
     }