Defer and "anonymous closure argument not contained in a closure" error

Using Swift
1

Sometimes I get really baffled by Swift's compiler.
Why doesn't B compile?
Why can't I put an anonymous closure argument inside a defer block?

struct Example {
	var value = 0
	
	mutating func doSomething( _: (inout Self) -> () ) {
		/* .... */
	}
}

var example = Example()

// A: OK
example.doSomething {
	$0.value = 1
	/* .... */
}

// B: ERROR!
example.doSomething {
	let saveValue = $0.value
	defer {
		$0.value = saveValue	// Anonymous closure argument not contained in a closure
	}	
	$0.value = 1
	/* .... */
}

// C: OK, so the issue seems to be just defer
example.doSomething {
	do {
		$0.value = 1
		/* .... */
	}
}

// D: OK
example.doSomething { examp in
	let saveValue = examp.value
	defer {
		examp.value = saveValue
	}
	examp.value = 1
	/* .... */
}
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