macOSだと問題ないね。
$ pbpaste let data = """ {"data": 9223372036854775808} """ struct Foo: Codable { var data: UInt64 } import Foundation print(try! JSONDecoder().decode(Foo.self, from: data.data(using: .utf8)!))% $ pbpaste|xcrun --toolchain org.swift.40320171205a swift - Foo(data: 9223372036854775808) $ pbpaste|xcrun --toolchain org.swift.41320180727a swift - Foo(data: 9223372036854775808) $ pbpaste|xcrun --toolchain org.swift.42120181030a swift - Foo(data: 9223372036854775808) $ pbpaste|xcrun --toolchain org.swift.5020190116a swift - Foo(data: 9223372036854775808)