"cast w/o mana cost" v. addnl cost/restrictions

[Korath]

Describe the Bug:
Effects that let you cast spells without paying their mana costs let you cast roughly half of the cards that have additional costs or with restrictions on whether you can cast them (including all such spells that aren't enchantments, instants (including flash), interrupts, or sorceries).

Fixture: Aluren on the bf, a Serra Avenger and a Skaab Ruinator in your hand, nothing in your graveyard, during your first turn.
Observed: You can "activate" the Aluren to cast the Serra Avenger, and again to cast the Skaab Ruinator.

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
dev 58bc91e3e.

What exactly should be the correct behavior/interaction?
Expected: Neither of those can be cast - the Serra Avenger because you can't cast it during your first, second, or third turns; the Skaab Ruinator because you don't have three creature cards in your graveyard to exile. Aluren and I think all such effects say they only let you cast spells without paying "their mana costs", not "their costs", and don't override "you can't cast this spell" text.

Are any other cards possibly affected by this bug?
Oh yes. Almost every single use of infinite_casting_cost() is incorrect; the only exceptions are where it's actually used to denote an unpayable casting cost, such as in a few corner cases of hybrid mana.

Aluren specifically will also fail for creature cards that use the correct method of returning 0 from EVENT_CAN_CAST (which currently only works for enchantment creatures and creatures with flash), since it doesn't even try to check if they're castable. I expect most such effects do the same, at least those that only let you cast permanents.