DIP30, delegates more destruction for your pleasure

[deadalnix]

On Thursday, 14 March 2013 at 03:45:39 UTC, kenji hara wrote:
> 2013/3/14 deadalnix <deadalnix at gmail.com>
>>
>> It is a tempting idea, but do not work. Consider :
>>
>> void delegate() immutable a;
>> const void delegate() b = a;
>>
>> This would be forbidden as covariance of function parameters 
>> and
>> transitivity act in opposite directions.
>>
>
> You are misunderstanding about delegate type syntax and 
> transitivity.
>
> 1. This qualifier for delegate type should be added after 
> parameter types.
>
> const void delegate() b;   // bad, typeof(b) == const(void 
> deelgate())
> void delegate() const b;   // good, b can hold const method
>

After or before don't change anything here as type qualifier are 
transitives. It means that a const delegate must have a const 
context. Or, in code :

static assert(is(const delegate() == const delegate() const)); // 
Pass

> 2. A delegate which has immutable context cannot implicitly 
> convertible to
> const context delegate.
>
> In general, if a delegate can implicitly convertible to 
> another, there
> should be _contravariance_ of parameter types.
>
> void delegate() immutable a;
> void delegate() const b;
> b = a;  // bad
> a = b;  // good
>

As showed above, this is equivalent to my sample code in the 
previous post, and it does break transitivity.

void delegate() const b; // Context of b is const (ie mutable or 
immutable).
void delegate() immutable a; // Context of a is immutable
a = b; // a now contains a reference to possibly mutable data 
considered as immutable by the type system.

This is *VERY* wrong.