syntax error, unexpected identifier "DATEDAY_TIME_DEFAULT_TYPE_FORMAT", expecting "=" in Composer\Autoload\{closure}() (line 32 of /src/Plugin/Field/FieldType/DateTimeDayItem.php

This error occurred for me when trying to add a new field to a content type. I removed the 'string' type declaration for these constants:

Before:

class DateTimeDayItem extends DateTimeItem {

  /**
   * Values for the 'time_type' setting: store only a time, time & seconds.
   */
  const string DATEDAY_TIME_DEFAULT_TYPE_FORMAT = 'time';
  const string DATE_TIME_DAY_H_I_FORMAT_STORAGE_FORMAT = 'H:i';
  const string DATEDAY_TIME_TYPE_SECONDS_FORMAT = 'time_seconds';
  const string DATE_TIME_DAY_H_I_S_FORMAT_STORAGE_FORMAT = 'H:i:s';

After:

class DateTimeDayItem extends DateTimeItem {

  /**
   * Values for the 'time_type' setting: store only a time, time & seconds.
   */
  const DATEDAY_TIME_DEFAULT_TYPE_FORMAT = 'time';
  const DATE_TIME_DAY_H_I_FORMAT_STORAGE_FORMAT = 'H:i';
  const DATEDAY_TIME_TYPE_SECONDS_FORMAT = 'time_seconds';
  const DATE_TIME_DAY_H_I_S_FORMAT_STORAGE_FORMAT = 'H:i:s';