diff -r 3d4e9c994f10 -r a86126ab1dd4 wp/wp-admin/includes/class-theme-installer-skin.php
--- a/wp/wp-admin/includes/class-theme-installer-skin.php	Tue Oct 22 16:11:46 2019 +0200
+++ b/wp/wp-admin/includes/class-theme-installer-skin.php	Tue Dec 15 13:49:49 2020 +0100
@@ -18,37 +18,78 @@
 class Theme_Installer_Skin extends WP_Upgrader_Skin {
 	public $api;
 	public $type;
+	public $url;
+	public $overwrite;
+
+	private $is_downgrading = false;
 
 	/**
 	 * @param array $args
 	 */
 	public function __construct( $args = array() ) {
 		$defaults = array(
-			'type'  => 'web',
-			'url'   => '',
-			'theme' => '',
-			'nonce' => '',
-			'title' => '',
+			'type'      => 'web',
+			'url'       => '',
+			'theme'     => '',
+			'nonce'     => '',
+			'title'     => '',
+			'overwrite' => '',
 		);
 		$args     = wp_parse_args( $args, $defaults );
 
-		$this->type = $args['type'];
-		$this->api  = isset( $args['api'] ) ? $args['api'] : array();
+		$this->type      = $args['type'];
+		$this->url       = $args['url'];
+		$this->api       = isset( $args['api'] ) ? $args['api'] : array();
+		$this->overwrite = $args['overwrite'];
 
 		parent::__construct( $args );
 	}
 
 	/**
+	 * Action to perform before installing a theme.
+	 *
+	 * @since 2.8.0
 	 */
 	public function before() {
 		if ( ! empty( $this->api ) ) {
-			$this->upgrader->strings['process_success'] = sprintf( $this->upgrader->strings['process_success_specific'], $this->api->name, $this->api->version );
+			$this->upgrader->strings['process_success'] = sprintf(
+				$this->upgrader->strings['process_success_specific'],
+				$this->api->name,
+				$this->api->version
+			);
 		}
 	}
 
 	/**
+	 * Hides the `process_failed` error when updating a theme by uploading a zip file.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @param WP_Error $wp_error WP_Error.
+	 * @return bool
+	 */
+	public function hide_process_failed( $wp_error ) {
+		if (
+			'upload' === $this->type &&
+			'' === $this->overwrite &&
+			$wp_error->get_error_code() === 'folder_exists'
+		) {
+			return true;
+		}
+
+		return false;
+	}
+
+	/**
+	 * Action to perform following a single theme install.
+	 *
+	 * @since 2.8.0
 	 */
 	public function after() {
+		if ( $this->do_overwrite() ) {
+			return;
+		}
+
 		if ( empty( $this->upgrader->result['destination_name'] ) ) {
 			return;
 		}
@@ -75,35 +116,59 @@
 		$install_actions = array();
 
 		if ( current_user_can( 'edit_theme_options' ) && current_user_can( 'customize' ) ) {
-			$customize_url               = add_query_arg(
+			$customize_url = add_query_arg(
 				array(
 					'theme'  => urlencode( $stylesheet ),
 					'return' => urlencode( admin_url( 'web' === $this->type ? 'theme-install.php' : 'themes.php' ) ),
 				),
 				admin_url( 'customize.php' )
 			);
-			$install_actions['preview']  = '<a href="' . esc_url( $customize_url ) . '" class="hide-if-no-customize load-customize">';
-			$install_actions['preview'] .= '<span aria-hidden="true">' . __( 'Live Preview' ) . '</span>';
-			/* translators: %s: theme name */
-			$install_actions['preview'] .= '<span class="screen-reader-text">' . sprintf( __( 'Live Preview &#8220;%s&#8221;' ), $name ) . '</span></a>';
+
+			$install_actions['preview'] = sprintf(
+				'<a href="%s" class="hide-if-no-customize load-customize">' .
+				'<span aria-hidden="true">%s</span><span class="screen-reader-text">%s</span></a>',
+				esc_url( $customize_url ),
+				__( 'Live Preview' ),
+				/* translators: %s: Theme name. */
+				sprintf( __( 'Live Preview &#8220;%s&#8221;' ), $name )
+			);
 		}
-		$install_actions['activate']  = '<a href="' . esc_url( $activate_link ) . '" class="activatelink">';
-		$install_actions['activate'] .= '<span aria-hidden="true">' . __( 'Activate' ) . '</span>';
-		/* translators: %s: theme name */
-		$install_actions['activate'] .= '<span class="screen-reader-text">' . sprintf( __( 'Activate &#8220;%s&#8221;' ), $name ) . '</span></a>';
+
+		$install_actions['activate'] = sprintf(
+			'<a href="%s" class="activatelink">' .
+			'<span aria-hidden="true">%s</span><span class="screen-reader-text">%s</span></a>',
+			esc_url( $activate_link ),
+			__( 'Activate' ),
+			/* translators: %s: Theme name. */
+			sprintf( _x( 'Activate &#8220;%s&#8221;', 'theme' ), $name )
+		);
 
 		if ( is_network_admin() && current_user_can( 'manage_network_themes' ) ) {
-			$install_actions['network_enable'] = '<a href="' . esc_url( wp_nonce_url( 'themes.php?action=enable&amp;theme=' . urlencode( $stylesheet ), 'enable-theme_' . $stylesheet ) ) . '" target="_parent">' . __( 'Network Enable' ) . '</a>';
+			$install_actions['network_enable'] = sprintf(
+				'<a href="%s" target="_parent">%s</a>',
+				esc_url( wp_nonce_url( 'themes.php?action=enable&amp;theme=' . urlencode( $stylesheet ), 'enable-theme_' . $stylesheet ) ),
+				__( 'Network Enable' )
+			);
 		}
 
-		if ( $this->type == 'web' ) {
-			$install_actions['themes_page'] = '<a href="' . self_admin_url( 'theme-install.php' ) . '" target="_parent">' . __( 'Return to Theme Installer' ) . '</a>';
+		if ( 'web' === $this->type ) {
+			$install_actions['themes_page'] = sprintf(
+				'<a href="%s" target="_parent">%s</a>',
+				self_admin_url( 'theme-install.php' ),
+				__( 'Return to Theme Installer' )
+			);
 		} elseif ( current_user_can( 'switch_themes' ) || current_user_can( 'edit_theme_options' ) ) {
-			$install_actions['themes_page'] = '<a href="' . self_admin_url( 'themes.php' ) . '" target="_parent">' . __( 'Return to Themes page' ) . '</a>';
+			$install_actions['themes_page'] = sprintf(
+				'<a href="%s" target="_parent">%s</a>',
+				self_admin_url( 'themes.php' ),
+				__( 'Return to Themes page' )
+			);
 		}
 
 		if ( ! $this->result || is_wp_error( $this->result ) || is_network_admin() || ! current_user_can( 'switch_themes' ) ) {
 			unset( $install_actions['activate'], $install_actions['preview'] );
+		} elseif ( get_option( 'template' ) === $stylesheet ) {
+			unset( $install_actions['activate'] );
 		}
 
 		/**
@@ -121,4 +186,199 @@
 			$this->feedback( implode( ' | ', (array) $install_actions ) );
 		}
 	}
+
+	/**
+	 * Check if the theme can be overwritten and output the HTML for overwriting a theme on upload.
+	 *
+	 * @since 5.5.0
+	 *
+	 * @return bool Whether the theme can be overwritten and HTML was outputted.
+	 */
+	private function do_overwrite() {
+		if ( 'upload' !== $this->type || ! is_wp_error( $this->result ) || 'folder_exists' !== $this->result->get_error_code() ) {
+			return false;
+		}
+
+		$folder = $this->result->get_error_data( 'folder_exists' );
+		$folder = rtrim( $folder, '/' );
+
+		$current_theme_data = false;
+		$all_themes         = wp_get_themes( array( 'errors' => null ) );
+
+		foreach ( $all_themes as $theme ) {
+			$stylesheet_dir = wp_normalize_path( $theme->get_stylesheet_directory() );
+
+			if ( rtrim( $stylesheet_dir, '/' ) !== $folder ) {
+				continue;
+			}
+
+			$current_theme_data = $theme;
+		}
+
+		$new_theme_data = $this->upgrader->new_theme_data;
+
+		if ( ! $current_theme_data || ! $new_theme_data ) {
+			return false;
+		}
+
+		echo '<h2 class="update-from-upload-heading">' . esc_html( __( 'This theme is already installed.' ) ) . '</h2>';
+
+		// Check errors for current theme.
+		if ( is_wp_error( $current_theme_data->errors() ) ) {
+			$this->feedback( 'current_theme_has_errors', $current_theme_data->errors()->get_error_message() );
+		}
+
+		$this->is_downgrading = version_compare( $current_theme_data['Version'], $new_theme_data['Version'], '>' );
+
+		$is_invalid_parent = false;
+		if ( ! empty( $new_theme_data['Template'] ) ) {
+			$is_invalid_parent = ! in_array( $new_theme_data['Template'], array_keys( $all_themes ), true );
+		}
+
+		$rows = array(
+			'Name'        => __( 'Theme name' ),
+			'Version'     => __( 'Version' ),
+			'Author'      => __( 'Author' ),
+			'RequiresWP'  => __( 'Required WordPress version' ),
+			'RequiresPHP' => __( 'Required PHP version' ),
+			'Template'    => __( 'Parent theme' ),
+		);
+
+		$table  = '<table class="update-from-upload-comparison"><tbody>';
+		$table .= '<tr><th></th><th>' . esc_html( __( 'Current' ) ) . '</th><th>' . esc_html( __( 'Uploaded' ) ) . '</th></tr>';
+
+		$is_same_theme = true; // Let's consider only these rows.
+
+		foreach ( $rows as $field => $label ) {
+			$old_value = $current_theme_data->display( $field, false );
+			$old_value = $old_value ? (string) $old_value : '-';
+
+			$new_value = ! empty( $new_theme_data[ $field ] ) ? (string) $new_theme_data[ $field ] : '-';
+
+			if ( $old_value === $new_value && '-' === $new_value && 'Template' === $field ) {
+				continue;
+			}
+
+			$is_same_theme = $is_same_theme && ( $old_value === $new_value );
+
+			$diff_field     = ( 'Version' !== $field && $new_value !== $old_value );
+			$diff_version   = ( 'Version' === $field && $this->is_downgrading );
+			$invalid_parent = false;
+
+			if ( 'Template' === $field && $is_invalid_parent ) {
+				$invalid_parent = true;
+				$new_value     .= ' ' . __( '(not found)' );
+			}
+
+			$table .= '<tr><td class="name-label">' . $label . '</td><td>' . wp_strip_all_tags( $old_value ) . '</td>';
+			$table .= ( $diff_field || $diff_version || $invalid_parent ) ? '<td class="warning">' : '<td>';
+			$table .= wp_strip_all_tags( $new_value ) . '</td></tr>';
+		}
+
+		$table .= '</tbody></table>';
+
+		/**
+		 * Filters the compare table output for overwriting a theme package on upload.
+		 *
+		 * @since 5.5.0
+		 *
+		 * @param string $table              The output table with Name, Version, Author, RequiresWP, and RequiresPHP info.
+		 * @param array  $current_theme_data Array with current theme data.
+		 * @param array  $new_theme_data     Array with uploaded theme data.
+		 */
+		echo apply_filters( 'install_theme_overwrite_comparison', $table, $current_theme_data, $new_theme_data );
+
+		$install_actions = array();
+		$can_update      = true;
+
+		$blocked_message  = '<p>' . esc_html( __( 'The theme cannot be updated due to the following:' ) ) . '</p>';
+		$blocked_message .= '<ul class="ul-disc">';
+
+		$requires_php = isset( $new_theme_data['RequiresPHP'] ) ? $new_theme_data['RequiresPHP'] : null;
+		$requires_wp  = isset( $new_theme_data['RequiresWP'] ) ? $new_theme_data['RequiresWP'] : null;
+
+		if ( ! is_php_version_compatible( $requires_php ) ) {
+			$error = sprintf(
+				/* translators: 1: Current PHP version, 2: Version required by the uploaded theme. */
+				__( 'The PHP version on your server is %1$s, however the uploaded theme requires %2$s.' ),
+				phpversion(),
+				$requires_php
+			);
+
+			$blocked_message .= '<li>' . esc_html( $error ) . '</li>';
+			$can_update       = false;
+		}
+
+		if ( ! is_wp_version_compatible( $requires_wp ) ) {
+			$error = sprintf(
+				/* translators: 1: Current WordPress version, 2: Version required by the uploaded theme. */
+				__( 'Your WordPress version is %1$s, however the uploaded theme requires %2$s.' ),
+				get_bloginfo( 'version' ),
+				$requires_wp
+			);
+
+			$blocked_message .= '<li>' . esc_html( $error ) . '</li>';
+			$can_update       = false;
+		}
+
+		$blocked_message .= '</ul>';
+
+		if ( $can_update ) {
+			if ( $this->is_downgrading ) {
+				$warning = sprintf(
+					/* translators: %s: Documentation URL. */
+					__( 'You are uploading an older version of a current theme. You can continue to install the older version, but be sure to <a href="%s">back up your database and files</a> first.' ),
+					__( 'https://wordpress.org/support/article/wordpress-backups/' )
+				);
+			} else {
+				$warning = sprintf(
+					/* translators: %s: Documentation URL. */
+					__( 'You are updating a theme. Be sure to <a href="%s">back up your database and files</a> first.' ),
+					__( 'https://wordpress.org/support/article/wordpress-backups/' )
+				);
+			}
+
+			echo '<p class="update-from-upload-notice">' . $warning . '</p>';
+
+			$overwrite = $this->is_downgrading ? 'downgrade-theme' : 'update-theme';
+
+			$install_actions['overwrite_theme'] = sprintf(
+				'<a class="button button-primary update-from-upload-overwrite" href="%s" target="_parent">%s</a>',
+				wp_nonce_url( add_query_arg( 'overwrite', $overwrite, $this->url ), 'theme-upload' ),
+				__( 'Replace current with uploaded' )
+			);
+		} else {
+			echo $blocked_message;
+		}
+
+		$cancel_url = add_query_arg( 'action', 'upload-theme-cancel-overwrite', $this->url );
+
+		$install_actions['themes_page'] = sprintf(
+			'<a class="button" href="%s" target="_parent">%s</a>',
+			wp_nonce_url( $cancel_url, 'theme-upload-cancel-overwrite' ),
+			__( 'Cancel and go back' )
+		);
+
+		/**
+		 * Filters the list of action links available following a single theme installation
+		 * failure when overwriting is allowed.
+		 *
+		 * @since 5.5.0
+		 *
+		 * @param string[] $install_actions Array of theme action links.
+		 * @param object   $api             Object containing WordPress.org API theme data.
+		 * @param array    $new_theme_data  Array with uploaded theme data.
+		 */
+		$install_actions = apply_filters( 'install_theme_overwrite_actions', $install_actions, $this->api, $new_theme_data );
+
+		if ( ! empty( $install_actions ) ) {
+			printf(
+				'<p class="update-from-upload-expired hidden">%s</p>',
+				__( 'The uploaded file has expired. Please go back and upload it again.' )
+			);
+			echo '<p class="update-from-upload-actions">' . implode( ' ', (array) $install_actions ) . '</p>';
+		}
+
+		return true;
+	}
 }