YANG when condition evaluated as false combined with a mandatory leaf does not work

[olofhagsand]

A construct with a conditiona (using when) container with a mandatory leaf does not work when the when condition evaluates as false.
Example:

container c {
   when ../id='true';
   leaf x {
      type string;
      mandatory true;
   }

If the when condition evaluates to false, x is still required at validation.
This fails RFC 7950 Sec 8.1:

The "mandatory" constraint is enforced for leafs and choices, unless the node
or any of its ancestors has a "when" condition ... that evaluates to "false".