Re: Evaluation order of "+="

[deadalnix via Digitalmars-d]

On Monday, 11 July 2016 at 23:04:00 UTC, Johan Engelen wrote:
LDC recently changed the evaluation order of "+=" (I think 
unintentionally, some other eval order problems were fixed). 
Now, it is different from DMD.
I am going to argue that I think DMD's order is more useful in 
the context of fibers, and would like your opinion.

Consider this code:
```
int sum;

int return1_add9tosum() {
    sum += 9;
    return 1;
}

void main() {
    sum = 0;
    sum += return1_add9tosum();

    import std.stdio;
    writeln(sum);
}
```
DMD 2.071 prints "10".
LDC master prints "1". (LDC 1.0.0 prints "10")

I find the spec [1] to be unclear on this point, so which one 
is correct?

The bug was caught by code involving fibers. Instead of 
`return1_add9tosum`, a function `return1_yieldsFiber` is 
called, and multiple fibers write to `sum`. In that case, upon 
completing the "+=", an older version of `sum` is used to 
calculate the result. I think that it is best to do what DMD 
does, such that fibers can do "+=" without worrying about 
yields on the rhs.

[1] https://dlang.org/spec/expression.html

There was a very lenghty discussion about this in the past. DMD 
is correct on that one. The semantic is such as :

int plusEqual(ref int a, int b) {
  a = a + b;
  return a;
}