[wp-trac] [WordPress Trac] #64156: add_post_type_support() does not merge sub-properties for the same feature

[WordPress Trac]

#64156: add_post_type_support() does not merge sub-properties for the same feature
--------------------------+-----------------------------
 Reporter:  fabiankaegy   |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  General       |    Version:
 Severity:  normal        |   Keywords:
  Focuses:                |
--------------------------+-----------------------------
 = Summary =

 When `add_post_type_support()` is called multiple times for the same post
 type and feature with associative array arguments, the second call
 overwrites the first call's arguments instead of merging them. This
 prevents incremental addition of sub-properties to post type features.

 = Current Behavior =

 {{{
 add_post_type_support( 'page', 'editor', array(
     'default-mode' => 'template-locked',
 ) );

 add_post_type_support( 'page', 'editor', array(
     'block-comments' => true,
 ) );

 // Result: Only 'block-comments' is stored, 'default-mode' is lost
 var_dump( get_all_post_type_supports( 'page' )['editor'] );
 // array( array( 'block-comments' => true ) )
 }}}

 = Expected Behavior =

 Multiple calls should merge the array arguments, allowing different
 plugins/themes to add their own sub-properties without conflicting:

 {{{
 add_post_type_support( 'page', 'editor', array(
     'default-mode' => 'template-locked',
 ) );

 add_post_type_support( 'page', 'editor', array(
     'block-comments' => true,
 ) );

 // Expected: Both properties should be preserved
 var_dump( get_all_post_type_supports( 'page' )['editor'] );
 // array( array( 'default-mode' => 'template-locked', 'block-comments' =>
 true ) )
 }}}

 = Technical Details =

 The issue is in `/src/wp-includes/post.php` at line 2292 (as of [xxxxx]),
 where the function simply overwrites the existing value:

 {{{
 if ( $args ) {
     $_wp_post_type_features[ $post_type ][ $feature ] = $args;
 }
 }}}

 = Proposed Solution =

 Detect when both the existing and new values are associative arrays and
 merge them using `array_merge()`. This maintains backwards compatibility
 because:

 - If the feature doesn't exist yet, it's created as before
 - If the feature is set to `true` (no args), calling with args still
 overwrites it as expected
 - Only when both values are arrays does merging occur
 - Non-associative array arguments continue to work as before

 = Use Cases =

 This is particularly useful for:

 1. **Block editor settings** - Different plugins adding different editor
 configuration options
 2. **Custom feature flags** - Themes and plugins progressively enhancing
 the same feature
 3. **Multi-plugin compatibility** - Avoiding conflicts when multiple
 plugins extend the same post type feature

 = Impact =

 This change would improve the API's usability and prevent unexpected
 behavior when multiple plugins/themes interact with the same post type
 features. The fix maintains full backwards compatibility.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/64156>
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