If a creature is both attacking and blocking in the same combat due to a rule change, what happens?

Under the Comprehensive Rules, attacking and blocking are defined as distinct, mutually exclusive roles within a single combat phase. CR 508.1 states that the active player declares attackers from among creatures they control, and CR 509.1 states that the defending player declares blockers. A creature already declared as an attacker is an 'attacking creature' for the duration of combat and cannot simultaneously be declared as a blocker.

CR 509.1b specifically restricts which creatures may be declared as blockers. Among the conditions: a creature cannot block if it is already attacking. This is reinforced by CR 506.4, which defines that a creature 'becomes an attacking creature' upon legal declaration and retains that status until it leaves combat. There is no standard rule mechanism that allows a single creature to occupy both roles at once.

No official Magic rule, card, or effect in the game's history allows a creature to simultaneously be both an attacker and a blocker in the same combat. If a hypothetical rule change were to attempt this, it would create an irresolvable conflict with core combat structure rules (CR 506–511), and the game would default to the existing framework where these roles are mutually exclusive.

Example: You control a 3/3 creature and declare it as an attacker targeting your opponent. Even if your opponent tries to find a loophole, that creature cannot also be declared as a blocker against a creature attacking you — it is locked into the attacking role for this combat phase.

In short: no card or rule in Magic permits this dual role, and the Comprehensive Rules are designed to prevent it. Any scenario positing this is either hypothetical or a misreading of how combat works.