Bug? Const Class Makes Constructor Const

[Steven Schveighoffer]

On 10/25/18 4:14 PM, Vijay Nayar wrote:
> On Thursday, 25 October 2018 at 19:51:45 UTC, Steven Schveighoffer wrote:
>> On 10/25/18 3:23 PM, Vijay Nayar wrote:
>>> void main()
>>> {
>>>    const class Bob {
>>>      this() {
>>>        bob = null;
>>>      }
>>>      Bob bob;
>>>    }
>>>
>>>    Bob b = new Bob();
>>> }
>>>
>>> Compiling this program gives the following error:
>>>
>>> onlineapp.d(10): Error: constructor `onlineapp.main.Bob.this() const` 
>>> is not callable using argument types `()`
>>>
>>> I know that declaring a 'const class' adds 'const' to every member of 
>>> the class, but does that even include constructors? I'm not sure how 
>>> one could use a class with a const constructor. Is this a bug?
>>
>> Generally:
>>
>> const(Bob) /*or auto*/ b = new const(Bob)();
>>
>> But the error message there looks really confusing to me.
>>
> 
> Yeah it's a bit odd.  The error message isn't super clear, the only 
> reason I think it's because "const" is applying to the constructor, is 
> because I get the same error by manually applying const to all members 
> like below.
> 
> void main()
> {
>    class Bob {
>      this() const {
>        bob = null;
>      }
>      const Bob bob;
>    }
> 
>    auto b = new Bob();
> }
> 
> But if the "this() const {" is changed to "this() {", then the error 
> goes away.

No, I mean the correct way to construct an object with a const 
constructor is `new const(Bob)`. The error message seems to suggest you 
just aren't calling it with the right parameters, but subtly it is 
conveying the error:

Error: constructor `onlineapp.main.Bob.this() const` is not callable 
using argument types `()`

See, you should be calling it with argument types `() const`, but you 
are using `()`.

It's a terrible way to say "you can only call this on a const object", 
but constructors are special, so I think a special error message should 
be used.

To answer your other question -- yes it is correct that const is 
applying to the constructor when you apply it to the type.

-Steve