Issue 1974 - What's your opinion?

[Zoadian]

On Thursday, 24 October 2019 at 08:02:28 UTC, RazvanN wrote:
> Hello everyone,
>
> I am currently trying to solve old issues but some of them I'm 
> not entirely sure how to handle. One of them is 1974 [1]. 
> Here's the thing:
>
> struct Foo { int y; void opAddAssign(int z) { y += z; } }
> struct Bar { Foo fou; Foo test() { return fou; } }
> void main() {
>   Bar x;
>   x.fou.y = 0;
>   x.test() += 1337;
>   writefln(x.fou.y);
> }
>
> The problem is that  x.test() += 1337; calls opAddAssign on a 
> temporary and the person that filed the bug report claimed that 
> this should be an error since the call is useless (in this 
> case).
>
> I have responded to the issue with the following:
>
> "This is a bit problematic because there are 2 points of view 
> on this:
>
> 1. From a high-level point of view, the bug report is valid, 
> you are trying to assign to an rvalue (x.test() = x.test() + 
> 1337) which should an error.
>
> 2. From a lowering point of view you are basically calling the 
> member function of a temporary object 
> (x.test.opAddAssign(1337)), which by the current rules is valid 
> behavior.
>
> I guess that what could be implemented is something along the 
> lines of: if the opAssign/op*Assign function is pure, then you 
> can error/warn because the call has no effect, otherwise it is 
> possible that the assign function has side effects so calling 
> it is correct."
>
> What are your thoughts on this? Should be close the issue as an 
> invalid one?
>
> Cheers,
> RazvanN
>
> [1] https://issues.dlang.org/show_bug.cgi?id=1974

"if the opAssign/op*Assign function is pure, then you can 
error/warn because the call has no effect, otherwise it is 
possible that the assign function has side effects so calling it 
is correct."

Thats what I would expect it to do. And it improves on the 
current behaviour. If need be we can still restrict it more in 
the future (maybe enforce that rvalues have to be consumed?).