Capability to edit own post, but not create new one

Question

I have the same requirement. I believe it’s a bug.
I have today raised a WordPress TRAC reporting my findings.

https://core.trac.wordpress.org/ticket/29714

Have a look and see if you’ve got the same problem.

Workaround

I have also developed a workaround that may allow you to make progress in the mean time.

Create a filter function for “user_has_cap”.
In that filter function, when the capability being checked for is that which you have defined ( e.g. “edit_your_cpts”), add the following logic.

global $pagenow;
if ( $pagenow == "edit.php" && isset( $_REQUEST['post_type'] ) ) {
        $pagenow .= '?post_type=" . $_REQUEST["post_type' ];
}

This will then allow the failing function ( user_can_access_admin_page() ) to return true.

Update 11th October. The first part of the workaround had some unwanted side effects. To overcome these you need some more code.

add_filter( "add_menu_classes", "oik_sites_add_menu_classes" );
/**
 * Implement "add_menu_classes" filter for oik-sites
 *
 * Part 2 of the Workaround for TRAC #29714  
 */
function oik_sites_add_menu_classes( $menu ) {
  global $pagenow;
  if ( false !== strpos( $pagenow, "edit.php" ) ) {
    $pagenow = "edit.php";
  }
  return( $menu );
}