Evaluation order of "+="

[Johan Engelen via Digitalmars-d]

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