reference variables don't exist, but can simulate them

[NonNull]

On Sunday, 28 June 2020 at 21:01:36 UTC, NonNull wrote:
> On Sunday, 28 June 2020 at 20:59:59 UTC, NonNull wrote:
>> Using gdc (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
>> Please criticize:
>>
>> struct refer(T) {
>>   T* ptr;
>>   this(ref T x) { ptr = &x; }
>>   ref T _() { return *ptr; }
>>   alias _ this;
>>   string toString() { import std.conv; return to!string(*ptr);
>>  }
>> }
>>
>> This will make a reference variable (simulation). [ toString() 
>> is just for writeln. ]
>>
>> void main() {
>>     int i = 100;
>>     refer!int j = i;
>>     j = 3;
>>     writeln(i);
>>     i = 100;
>>     writeln(j);
>>     j += 3;
>>     writeln(i);
>>     refer!int k = j;
>>     writeln(k);
>> }
>>
>> And a refer!int can be returned as it is just a value. 
>> Returning one that contains a pointer to a local variable 
>> leads to a compilation error.
>
> * does not lead to a compilation error

Now with a different compiler I this:

Deprecation: Cannot use alias this to partially initialize 
variable j of type refer. Use j._()

This is for the line j=3

What is this about? Where does this hidden rule come from?