Use constant as default type when adding new keys

features/config-set.feature

@@ -41,16 +41,16 @@ Feature: Set the value of a constant or variable defined in wp-config.php file
       variable_value
       """
 
-    When I try `wp config set SOME_NAME some_value`
-    Then STDERR should be:
+    When I run `wp config set DEFAULT_TO_CONSTANT some_value`
+    Then STDOUT should be:
       """
-      Error: The constant or variable 'SOME_NAME' is not defined in the 'wp-config.php' file. Specify an explicit --type=<type> to add.
+      Success: Added the constant 'DEFAULT_TO_CONSTANT' to the 'wp-config.php' file with the value 'some_value'.
       """
 
-    When I try `wp config get SOME_NAME`
-    Then STDERR should be:
+    When I run `wp config get DEFAULT_TO_CONSTANT`
+    Then STDOUT should be:
       """
-      Error: The constant or variable 'SOME_NAME' is not defined in the 'wp-config.php' file.
+      some_value
       """
 
   Scenario: Updating a non-existent value without --add

src/Config_Command.php

@@ -464,11 +464,12 @@ public function set( $args, $assoc_args ) {
 						WP_CLI::error( "Found both a constant and a variable '{$name}' in the 'wp-config.php' file. Use --type=<type> to disambiguate." );
 					}
 					if ( ! $has_constant && ! $has_variable ) {
-						$message = "The constant or variable '{$name}' is not defined in the 'wp-config.php' file.";
-						if ( $options['add'] ) {
-							$message .= ' Specify an explicit --type=<type> to add.';
+						if ( ! $options['add'] ) {
+							$message = "The constant or variable '{$name}' is not defined in the 'wp-config.php' file.";
+							WP_CLI::error( $message );
 						}
-						WP_CLI::error( $message );
+						$type   = 'constant';
+						$adding = true;
 					} else {
 						$type = $has_constant ? 'constant' : 'variable';
 					}