An 18-inch treaty standard could push Japan toward 20- or 22-inch guns

If a naval treaty allowed 18-inch guns, would Japan build Yamato with 20-inch weapons?

Potentially, and it might push as far as 22 inches, but the required ship could break the limits of Japanese infrastructure.

Yamato's design logic depends on carrying a weapon that foreign battleships cannot easily match. If a treaty standardizes 18-inch guns, merely adopting the same caliber no longer provides that advantage. Japan would therefore have an incentive to investigate 20- or even 22-inch weapons despite their low rate of fire. The resulting ship would need extraordinary displacement and could exceed the limits of Japanese yards and support infrastructure. A higher arms-control ceiling can accelerate competition rather than relax it.