Not expected result when changing a variable in a "defer" block, if the variable has observers willSet or didSet.

I changed by adding a statement in willSet:

func foo() {
  var i = 0 {
    willSet { print("willSet", i) }
  }
   
  defer {
    i -= 1
    print("defer", i) // expected result - 2, prints - 3
  }
   
  i = 3
}

foo()

And get the expected result willSet 0 willSet 3 defer 2

I instrumented more:

func foo() {
  var i = 0 {
    willSet { print("willSet, i is still", i) }
  }
   
  defer {
    i -= 1
    print("defer", i) // expected result - 2, prints - 3
  }
    
   print("soon i = 3, now is", i)
  i = 3
    print("now i = 3", i)
}

foo()

And get:

• soon i = 3, now is 0
• willSet, i is still 0
• now i = 3 3
• willSet, i is still 3
• defer 2

I remove the call to i in willSet:

    willSet { print("willSet") } // , i is still", i) }

And get:

• soon i = 3, now is 0
• willSet
• now i = 3 3
• willSet
• defer 3

Thanks for finding and sharing an interesting example.

For me, it seems to be a bug of Swift. You should better send a bug report to swift.org.

Effectively, exact same problem with didSet.

I tracked in debugger and I observe that willSet does not update i:

func foo() {
  var i = 0 {
     willSet { print("willSet") }
  }
   
  defer {
    i -= 1
    print("defer", i) // expected result - 2, prints - 3
  }
    
   print("soon i = 3, now is", i)
  i = 3
    print("now i = 3", i)
}

foo()

Here is the sequence and values in console:

Statement                     value in console

• var i = 0
• print("Soon")             i = 0
• i = 3                            value = 3
• willSet                        newValue = 3
• print("now")              i = 3
• defer
• i -= 1
• willSet                        value = 2
• willSet                        newValue = 2 // newValue is correctly passed
• willSet                        i = 3 // after executing willSet, i is NOT updated to newValue : that's the bug