relax inout rules?

[Timon Gehr]

On 12/13/2011 07:26 AM, Walter Bright wrote:
> On 12/12/2011 10:56 AM, Steven Schveighoffer wrote:
>> If this ends up being viable, this is actually easier to explain than the
>> current rules for inout. We just have to make sure the rules are sound
>> before
>> doing something like this.
>
> I don't understand the point of doing this. Just make it const.

It does not work with const. (the fact that it currently does with DMD 
is a severe bug).

Consider:

void bad(const(int)** x, const(int)* y){ *x=y; }

void main(){
     immutable(int) i = 1;
     int* x;
     bad(&x, &i); // should error int** does not convert to const(int)**
     // currently:
     assert(x is &i); // mutable reference to immutable data
     *x = 2; // BOOM!
}

On the other hand:

void main(){
     immutable(int) i = 1;
     immutable(int)* x;
     bad(&x, &i); // fine, but the type checker cannot know that
}

Therefore it is sensible to allow

void good(inout(int)** x, inout(int)* y){ *x=y; }

void main(){
     immutable(int) i = 1;
     int* x;
     good(&x, &i); // error, no value for inout makes this typecheck
}

On the other hand:

void good(inout(int)** x, inout(int)* y){ *x=y; }

void main(){
     immutable(int) i = 1;
     immutable(int)* x;
     good(&x, &i); // this can now typecheck: use inout as immutable
}