iOS Objective-C: weakSelf Syntax

Nearly every example of the weakSelf pattern I encounter online has the syntax

__weak typeof(self) weakSelf = self;

But the compiler complains about this syntax in the latest version of Xcode, requiring:

__weak __typeof__(self) weakSelf = self;

Did the syntax change, or am i missing an import? I tried pulling in <objc/runtime.h> but no change.

Stealthy bonus question: Why not just cast it explicitly?

__weak MyBoffoClass *weakSelf = self;

I found one answer here that said it was better to cast it directly, but no reasoning as to why.