Plus equals for setter does not work

[flattened]

    # foo.nim
    
    type
      Foo* = object
        bar: int
    
    proc `bar`*(f: var Foo): int {.inline.} =
      f.bar
    
    proc `bar=`*(f: var Foo, val: int) {.inline.} =
      f.bar = val
    
    
    Run
    
    
    # main.nim
    
    import foo
    
    var f: Foo
    f.bar += 1
    echo f.bar
    
    
    
    Run

Results in: 
    
    
    Error: type mismatch: got <int, int literal(1)>
    but expected one of:
    proc `+=`[T: SomeOrdinal | uint | uint64](x: var T; y: T)
      for a 'var' type a variable needs to be passed, but 'bar(f)' is immutable
    proc `+=`[T: float | float32 | float64](x: var T; y: T)
      first type mismatch at position: 1
      required type: var T: float or float32 or float64
      but expression 'bar(f)' is of type: int
      for a 'var' type a variable needs to be passed, but 'bar(f)' is immutable
    
    expression: bar(f) += 1
    
    
    Run