Trigger: "a spell or ability an opponent controls causes you to discard this card"

[philluiz2323]

Trigger: "When a spell or ability an opponent controls causes you to discard this card, [effect]"

Summary

A triggered ability that fires when an opponent's spell or ability causes you to
discard the card — "When a spell or ability an opponent controls causes you to
discard this card, [effect]" — is unparsed and drops the card to a gap.

15 single-gap cards: Guerrilla Tactics, Gorilla Tactics, Sand Golem, Quagnoth,
Mangara's Blessing, Pure Intentions, Ajani's Last Stand, Spiritual Focus
(whenever / "a card"), and more.

Verified non-duplicate

• Issues/PRs: "Guerrilla Tactics" → 0; the "causes you to discard this card" hits
  are all the unrelated Library of Leng discard-replacement ("put it on top
  instead"), not this trigger.
• Code: no trigger condition for "discarded by an opponent's spell/ability" in
  types/triggers.rs / oracle_trigger*.

Existing infrastructure

The replacement form of this exact event is already recognized —
oracle_replacement.rs:603 parses "If a spell or ability an opponent controls causes
you to discard this card, put it onto the battlefield instead …". So the engine
already has the notion of "an opponent's spell/ability caused this discard"; this issue
adds the triggered-ability counterpart ("When … , [effect]").

Proposed approach

1. Trigger condition (run add-engine-variant gate): add a TriggerCondition
   for "you discard this card because of a spell/ability an opponent controls" (CR
   603.2 + CR 701.8 discard). Mirror the predicate the replacement form already uses
   to identify the opponent-caused discard of the source card.
2. Trigger matcher: fire when the discard event's cause is an opponent-controlled
   spell/ability and the discarded object is the source (self). The discard event must
   carry (or be matchable to) the cause's controller.
3. Parser: recognize "When/Whenever a spell or ability an opponent controls causes
   you to discard [this card | a card], [effect]" → a TriggerDefinition with the new
   condition; parse the effect via the standard chain. Reject-and-gap unparseable
   effect bodies.

Scope

• First pass: the self-discard form ("…causes you to discard this card,
  [effect]") with controller-/source-scoped effects — Guerrilla Tactics (deals 4
  damage), Sand Golem / Quagnoth / Pure Intentions (return this card to hand),
  Mangara's Blessing (gain 2 life). Establishes the trigger end to end.
• Follow-ups: the "a card" form (Spiritual Focus — triggers on any discard this
  way, not just self) and intervening-if variants (Ajani's Last Stand "if you
  control…").

Why this is valuable

• 15 cards, one coherent trigger, genuinely non-duplicate, building on the
  existing opponent-caused-discard recognition (the replacement form).

Acceptance criteria

• An opponent's spell/ability discarding the card fires the ability (e.g. Guerrilla
  Tactics deals 4 damage); a self-caused discard (your own spell, or cycling) does
  not.
• Unsupported effect bodies stay a loud gap.
• add-engine-variant gate passed; CR verified; exhaustive matches updated.
• coverage-honesty / clippy / fmt / parser-combinator / engine-authorities green;
  regen + cargo test --test integration before pushing.

[mike-theDude]

Reviewed against current main: fixed by 5aa3c1b44. Discard events now retain their cause and controller, and the real-pipeline regression proves the "this card" trigger fires only when an opponent-controlled spell or ability caused the discard, not for self-caused or cause-less discards. This appears resolved; please close the issue.

[matthewevans]

Closing per mike-theDude's triage above — verified fixed/resolved on current main. If this still reproduces on a current build, please reopen with a fresh repro.