Q:

Why does SOQL allow filtering on List variable types?

Why does the following work?
List<Account> accounts = [SELECT Id, Name FROM Account LIMIT 3];

List<Contact> contacts = [SELECT Id 
                     FROM Contact
                     WHERE AccountId = :accounts];

System.debug(contacts);

First, this is working on a "list of ids" (not really, it's a list of sObjects, more on that in a second) without using the IN operator (i.e AccountId IN [...]).
Second, the list is not a list of ids, but rather a list of sObjects. How does SOQL know to pull the Id of the sObject?
I someone can point me to where this is documented, that'd be great.

A:

It is interesting that the documentation that I've found doesn't explicitly state this scenario is covered, at least in the main text, where it says:

The value of the IN or NOT IN operator in WHERE clauses, allowing filtering on a dynamic set of values. Note that this is of particular use with a list of IDs or Strings, though it works with lists of any type

That page's code example does, however, include the following comment:
// An IN-bind with an Id list. Note that a list of sObjects
// can also be used--the Ids of the objects are used for
// the bind

Suffice to say, SOQL from Apex is smart enough to convert your list of SObjects into a list of IDs for the execution of the query.

