Does Void Winnower prevent casting spells with even converted mana costs if their mana value changes due to X?

Void Winnower states that your opponents cannot cast spells with even converted mana costs (now called mana values). The critical question is: when is mana value determined, and what value does X have?

Under CR 202.3, the mana value of a spell on the stack is calculated using the actual mana paid, including the chosen value of X. However, the restriction from Void Winnower applies at the time you attempt to cast the spell — meaning you must be able to legally cast it in the first place. If X=0 (the minimum), the mana value may be even, blocking the cast entirely.

More precisely, CR 601.2 governs the process of casting a spell. You cannot legally begin casting a spell if no legal choices exist that would make it castable. If every possible value of X results in an even mana value, the spell cannot be cast at all. If at least one odd value of X is available and affordable, the spell can be cast — but only with that odd X value.

For example: your opponent controls Void Winnower and you want to cast Fireball (CMC of X+R). If you pay X=0, mana value = 1 (odd) — allowed. If you pay X=1, mana value = 2 (even) — blocked. So you can cast Fireball, but only with an odd value of X (0, 2 would be even counting the R, so X=0 gives MV=1, X=2 gives MV=3, etc.). Conversely, a spell like Emrakul, the Aeons Torn with no X has a fixed even mana value and simply cannot be cast.

In summary: Void Winnower does restrict X spells, but only if the chosen X results in an even total mana value. Players can often work around it by choosing an X that yields an odd mana value, provided they can pay for it.