ConditionInterface::compile() is being extended in error.

That solution, I prefer....

  /**
   * Compiles the saved conditions for later retrieval.
   *
   * This method does not return anything, but simply prepares data to be
   * retrieved via __toString() and arguments().
   *
   * @param $connection
   *   The database connection for which to compile the conditionals.
   * @param $queryPlaceholder
   *   The query this condition belongs to. If not given, the current query is
   *   used.
+ * 
+ * @return void
   */

There must be many other interface definitions which make a similar assumption about the return type ....

from https://www.drupal.org/coding-standards/docs
under the "Drupal API documentation standards for functions" I have this line

If there is no return value for a function, there must not be a @return tag.

It make me feel better, when I see that the developer before me explicitly inserted warning text about the return type in this instance.

The exception to the rule would have to be This is an interface definition, so normal function rules do not apply!

Anyway this needs review.

On a completely separate point, Core is trying to appear standardized while not actually being standardized.

Please note that \Drupal\Core\Entity\ConditionInterface also implements a compile() function and is its annotation is in error, in another direction.
The error is that the interface should indicate a return type, as functions that implement it do return a value.

I am still pondering what to do there...in what is not a database issue and the gate for API changes is closed.