Fix admin notice class.

class-tgm-plugin-activation.php

@@ -1180,12 +1180,7 @@ public function notices() {
 				}
 
 				// Register the nag messages and prepare them to be processed.
-				if ( ! empty( $this->strings['nag_type'] ) ) {
-					add_settings_error( 'tgmpa', 'tgmpa', $rendered, sanitize_html_class( strtolower( $this->strings['nag_type'] ) ) );
-				} else {
-					$nag_class = version_compare( $this->wp_version, '3.8', '<' ) ? 'updated' : 'update-nag';
-					add_settings_error( 'tgmpa', 'tgmpa', $rendered, $nag_class );
-				}
+				add_settings_error( 'tgmpa', 'tgmpa', $rendered, $this->get_admin_notice_class() );
 			}
 
 			// Admin options pages already output settings_errors, so this is to avoid duplication.
@@ -1194,6 +1189,30 @@ public function notices() {
 			}
 		}
 
+		/**
+		 * Get admin notice class.
+		 *
+		 * Work around all the changes to the various admin notice classes between WP 4.4 and 3.7
+		 * (lowest supported version by TGMPA).
+		 *
+		 * @since 2.x.x
+		 *
+		 * @return string
+		 */
+		protected function get_admin_notice_class() {
+			if ( ! empty( $this->strings['nag_type'] ) ) {
+				return sanitize_html_class( strtolower( $this->strings['nag_type'] ) );
+			} else {
+				if ( version_compare( $this->wp_version, '4.2', '>=' ) ) {
+					return 'notice-warning';
+				} elseif ( version_compare( $this->wp_version, '4.1', '>=' ) ) {
+					return 'notice';
+				} else {
+					return 'updated';
+				}
+			}
+		}
+
 		/**
 		 * Display settings errors and remove those which have been displayed to avoid duplicate messages showing
 		 *

example.php

@@ -191,7 +191,7 @@ function my_theme_register_required_plugins() {
 			'complete'                        => __( 'All plugins installed and activated successfully. %1$s', 'theme-slug' ), // %s = dashboard link.
 			'contact_admin'                   => __( 'Please contact the administrator of this site for help.', 'tgmpa' ),
 
-			'nag_type'                        => 'updated', // Determines admin notice type - can only be 'updated', 'update-nag' or 'error'.
+			'nag_type'                        => '', // Determines admin notice type - can only be one of the typical WP notice classes, such as 'updated', 'update-nag', 'notice-warning', 'notice-info' or 'error'. Some of which may not work as expected in older WP versions.
 		),
 		*/
 	);