The command /give @s diamond, which I used commonly in pre-17w45a, no longer works. It fails with the message "Only players may be affected by this command", even though @s can only target a player in this instance. If I change it to /give @s[type=player] diamond, it works.

Why this is a problem:
The game's assertion that @s could target entities other than players is false when the player is executing the command.