Registers theme support for a given feature.

Must be called in the theme’s functions.php file to work.
If attached to a hook, it must be {@see ‘after_setup_theme’}.
The {@see ‘init’} hook may be too late for some features.

Example usage:

add_theme_support( ‘title-tag’ );
add_theme_support( ‘custom-logo’, array(
‘height’ => 480,
‘width’ => 720,
) );

function add_theme_support( $feature, ...$args ) {
	global $_wp_theme_features;

	if ( ! $args ) {
		$args = true;
	}

	switch ( $feature ) {
		case 'post-thumbnails':
			// All post types are already supported.
			if ( true === get_theme_support( 'post-thumbnails' ) ) {
				return;
			}

			/*
			 * Merge post types with any that already declared their support
			 * for post thumbnails.
			 */
			if ( isset( $args[0] ) && is_array( $args[0] ) && isset( $_wp_theme_features['post-thumbnails'] ) ) {
				$args[0] = array_unique( array_merge( $_wp_theme_features['post-thumbnails'][0], $args[0] ) );
			}

			break;

		case 'post-formats':
			if ( isset( $args[0] ) && is_array( $args[0] ) ) {
				$post_formats = get_post_format_slugs();
				unset( $post_formats['standard'] );

				$args[0] = array_intersect( $args[0], array_keys( $post_formats ) );
			} else {
				_doing_it_wrong(
					"add_theme_support( 'post-formats' )",
					__( 'You need to pass an array of post formats.' ),
					'5.6.0'
				);
				return false;
			}
			break;

		case 'html5':
			// You can't just pass 'html5', you need to pass an array of types.
			if ( empty( $args[0] ) || ! is_array( $args[0] ) ) {
				_doing_it_wrong(
					"add_theme_support( 'html5' )",
					__( 'You need to pass an array of types.' ),
					'3.6.1'
				);

				if ( ! empty( $args[0] ) && ! is_array( $args[0] ) ) {
					return false;
				}

				// Build an array of types for back-compat.
				$args = array( 0 => array( 'comment-list', 'comment-form', 'search-form' ) );
			}

			// Calling 'html5' again merges, rather than overwrites.
			if ( isset( $_wp_theme_features['html5'] ) ) {
				$args[0] = array_merge( $_wp_theme_features['html5'][0], $args[0] );
			}
			break;

		case 'custom-logo':
			if ( true === $args ) {
				$args = array( 0 => array() );
			}
			$defaults = array(
				'width'                => null,
				'height'               => null,
				'flex-width'           => false,
				'flex-height'          => false,
				'header-text'          => '',
				'unlink-homepage-logo' => false,
			);
			$args[0]  = wp_parse_args( array_intersect_key( $args[0], $defaults ), $defaults );

			// Allow full flexibility if no size is specified.
			if ( is_null( $args[0]['width'] ) && is_null( $args[0]['height'] ) ) {
				$args[0]['flex-width']  = true;
				$args[0]['flex-height'] = true;
			}
			break;

		case 'custom-header-uploads':
			return add_theme_support( 'custom-header', array( 'uploads' => true ) );

		case 'custom-header':
			if ( true === $args ) {
				$args = array( 0 => array() );
			}

			$defaults = array(
				'default-image'          => '',
				'random-default'         => false,
				'width'                  => 0,
				'height'                 => 0,
				'flex-height'            => false,
				'flex-width'             => false,
				'default-text-color'     => '',
				'header-text'            => true,
				'uploads'                => true,
				'wp-head-callback'       => '',
				'admin-head-callback'    => '',
				'admin-preview-callback' => '',
				'video'                  => false,
				'video-active-callback'  => 'is_front_page',
			);

			$jit = isset( $args[0]['__jit'] );
			unset( $args[0]['__jit'] );

			// Merge in data from previous add_theme_support() calls.
			// The first value registered wins. (A child theme is set up first.)
			if ( isset( $_wp_theme_features['custom-header'] ) ) {
				$args[0] = wp_parse_args( $_wp_theme_features['custom-header'][0], $args[0] );
			}

			// Load in the defaults at the end, as we need to insure first one wins.
			// This will cause all constants to be defined, as each arg will then be set to the default.
			if ( $jit ) {
				$args[0] = wp_parse_args( $args[0], $defaults );
			}

			/*
			 * If a constant was defined, use that value. Otherwise, define the constant to ensure
			 * the constant is always accurate (and is not defined later,  overriding our value).
			 * As stated above, the first value wins.
			 * Once we get to wp_loaded (just-in-time), define any constants we haven't already.
			 * Constants are lame. Don't reference them. This is just for backward compatibility.
			 */

			if ( defined( 'NO_HEADER_TEXT' ) ) {
				$args[0]['header-text'] = ! NO_HEADER_TEXT;
			} elseif ( isset( $args[0]['header-text'] ) ) {
				define( 'NO_HEADER_TEXT', empty( $args[0]['header-text'] ) );
			}

			if ( defined( 'HEADER_IMAGE_WIDTH' ) ) {
				$args[0]['width'] = (int) HEADER_IMAGE_WIDTH;
			} elseif ( isset( $args[0]['width'] ) ) {
				define( 'HEADER_IMAGE_WIDTH', (int) $args[0]['width'] );
			}

			if ( defined( 'HEADER_IMAGE_HEIGHT' ) ) {
				$args[0]['height'] = (int) HEADER_IMAGE_HEIGHT;
			} elseif ( isset( $args[0]['height'] ) ) {
				define( 'HEADER_IMAGE_HEIGHT', (int) $args[0]['height'] );
			}

			if ( defined( 'HEADER_TEXTCOLOR' ) ) {
				$args[0]['default-text-color'] = HEADER_TEXTCOLOR;
			} elseif ( isset( $args[0]['default-text-color'] ) ) {
				define( 'HEADER_TEXTCOLOR', $args[0]['default-text-color'] );
			}

			if ( defined( 'HEADER_IMAGE' ) ) {
				$args[0]['default-image'] = HEADER_IMAGE;
			} elseif ( isset( $args[0]['default-image'] ) ) {
				define( 'HEADER_IMAGE', $args[0]['default-image'] );
			}

			if ( $jit && ! empty( $args[0]['default-image'] ) ) {
				$args[0]['random-default'] = false;
			}

			// If headers are supported, and we still don't have a defined width or height,
			// we have implicit flex sizes.
			if ( $jit ) {
				if ( empty( $args[0]['width'] ) && empty( $args[0]['flex-width'] ) ) {
					$args[0]['flex-width'] = true;
				}
				if ( empty( $args[0]['height'] ) && empty( $args[0]['flex-height'] ) ) {
					$args[0]['flex-height'] = true;
				}
			}

			break;

		case 'custom-background':
			if ( true === $args ) {
				$args = array( 0 => array() );
			}

			$defaults = array(
				'default-image'          => '',
				'default-preset'         => 'default',
				'default-position-x'     => 'left',
				'default-position-y'     => 'top',
				'default-size'           => 'auto',
				'default-repeat'         => 'repeat',
				'default-attachment'     => 'scroll',
				'default-color'          => '',
				'wp-head-callback'       => '_custom_background_cb',
				'admin-head-callback'    => '',
				'admin-preview-callback' => '',
			);

			$jit = isset( $args[0]['__jit'] );
			unset( $args[0]['__jit'] );

			// Merge in data from previous add_theme_support() calls. The first value registered wins.
			if ( isset( $_wp_theme_features['custom-background'] ) ) {
				$args[0] = wp_parse_args( $_wp_theme_features['custom-background'][0], $args[0] );
			}

			if ( $jit ) {
				$args[0] = wp_parse_args( $args[0], $defaults );
			}

			if ( defined( 'BACKGROUND_COLOR' ) ) {
				$args[0]['default-color'] = BACKGROUND_COLOR;
			} elseif ( isset( $args[0]['default-color'] ) || $jit ) {
				define( 'BACKGROUND_COLOR', $args[0]['default-color'] );
			}

			if ( defined( 'BACKGROUND_IMAGE' ) ) {
				$args[0]['default-image'] = BACKGROUND_IMAGE;
			} elseif ( isset( $args[0]['default-image'] ) || $jit ) {
				define( 'BACKGROUND_IMAGE', $args[0]['default-image'] );
			}

			break;

		// Ensure that 'title-tag' is accessible in the admin.
		case 'title-tag':
			// Can be called in functions.php but must happen before wp_loaded, i.e. not in header.php.
			if ( did_action( 'wp_loaded' ) ) {
				_doing_it_wrong(
					"add_theme_support( 'title-tag' )",
					sprintf(
						/* translators: 1: title-tag, 2: wp_loaded */
						__( 'Theme support for %1$s should be registered before the %2$s hook.' ),
						'title-tag',
						'wp_loaded'
					),
					'4.1.0'
				);

				return false;
			}
	}

	$_wp_theme_features[ $feature ] = $args;
}