return scope ref outlives the scope of the argument

[Eugene Wissner]

On Tuesday, 25 June 2019 at 11:16:47 UTC, Jonathan M Davis wrote:
> On Tuesday, June 25, 2019 1:32:58 AM MDT Eugene Wissner via 
> Digitalmars-d- learn wrote:
>> struct Container
>> {
>> }
>>
>> static struct Inserter
>> {
>>      private Container* container;
>>
>>      private this(return scope ref Container container) 
>> @trusted
>>      {
>>          this.container = &container;
>>      }
>>
>> }
>>
>> auto func()()
>> {
>>      Container container;
>>      return Inserter(container);
>> }
>>
>> void main()
>> {
>>      static assert(!is(typeof(func!())));
>> }
>>
>> The code above compiles with dmd 2.085, but not 2.086 (with
>> -preview=dip1000). What am I doing wrong?
>
> You're storing a pointer to a scope variable. That's violating 
> the entire point of scope. If something is scope, you can't 
> store any kind of reference to it. And since container is a 
> local variable in func, and Inserter tries to return from func 
> with a pointer to container, you definitely have an @safety 
> problem, because that pointer would be invalid once func 
> returned.
>
> - Jonathan M Davis

So you're saying that func() shouldn't compile? And it is exactly 
what the assertion in the main function does: it asserts that the 
function cannot be instantiated. And it was true for 2.085 but it 
can be instantiated with 2.086.