Parameter $on at Select::join() allow Zend\Db\Sql\Predicate\Expression

src/Sql/Join.php

@@ -110,7 +110,7 @@ public function getJoins()
     /**
      * @param string|array|TableIdentifier $name A table name on which to join, or a single
      *     element associative array, of the form alias => table, or TableIdentifier instance
-     * @param string $on A string specification describing the fields to join on.
+     * @param string|Predicate\Expression $on A specification describing the fields to join on.
      * @param string|string[]|int|int[] $columns A single column name, an array
      *     of column names, or (a) specification(s) such as SQL_STAR representing
      *     the columns to join.

src/Sql/Select.php

@@ -263,7 +263,7 @@ public function columns(array $columns, $prefixColumnsWithTable = true)
      * Create join clause
      *
      * @param  string|array|TableIdentifier $name
-     * @param  string $on
+     * @param  string|Predicate\Expression $on
      * @param  string|array $columns
      * @param  string $type one of the JOIN_* constants
      * @return self Provides a fluent interface