__typeid

[Bruce Carneal]

On Saturday, 1 August 2020 at 18:26:54 UTC, Andrei Alexandrescu 
wrote:
> On 8/1/20 1:29 PM, Jacob Carlborg wrote:
>> On 2020-08-01 18:12, Andrei Alexandrescu wrote:
>>> Progress is moving along nicely with __typeid.
>> 
>> I recommend __typeid to be private and have the compiler 
>> lowering to __traits(getMember, object, "__typeid") to bypass 
>> private. This is to make sure this symbol is not accessed 
>> directly.
>
> That's related to the matter of immutability. Currently all 
> __typeid objects are immutable and live in static read-only 
> storage. That's nice and would make it impossible to mess with 
> them. However, that makes them very unpleasant to use, e.g. you 
> can't use them as out parameters, can't assign them etc. All 
> that old story with tail const.

That unpleasantness, and the attendant complexities of the 
current workarounds, motivates Stefan's type function work.  I 
agree with beerconf Atila, IIUC, that a good solution to this 
will probably involve a generalization to some form of restricted 
type variables.  Perhaps mutable forms that are canonized to 
unique? immutables.