dip1000 and preview in combine to cause extra safety errors

[Timon Gehr]

On 09.06.22 01:19, Walter Bright wrote:
> On 6/8/2022 10:50 AM, John Colvin wrote:
>> The problem is `foo` and whether the compiler should somehow prevent 
>> the inconsistency between the signature and implementation. Obviously 
>> the answer is “yes, ideally”, but in practice with @safe, @system, 
>> dip1000, @live and so on it’s all a mess.
> 
> The checks aren't done for @system code. Yes, the compiler believes you 
> for @system code. It's the point of @system code.
> 
> If foo() is annotated with @safe,
> 
>    test6.d(5): Deprecation: scope variable `s` may not be returned
> 
> The compiler is working as intended, this is not unexpected behavior.

Actually it *is* unexpected behavior.

```d

int* foo()@system{
     int x;
     return &x; // error
}

int* foo(ref int x)@system{
     return &x; // error
}

int* foo(scope int* x)@system{
     return x; // ok
}
```

This does not have anything to do with `@safe` by default, it's just an 
inconsistency in the compiler implementation.