Struct inheritance

[Richard (Rikki) Andrew Cattermole]

On 06/12/2024 2:04 PM, Jonathan M Davis wrote:
> On Thursday, December 5, 2024 5:35:38 PM MST Richard (Rikki) Andrew Cattermole
> via dip.ideas wrote:
>> For the checking of inheritance what I mean is:
>>
>> ``is(Child : Parent)``
> 
> Except that that _doesn't_ check for inheritance. It checks for an implicit
> conversion. It's just used for checking for inheritance based on the fact
> that a reference to a child class can implicitly convert to a reference of
> its parent class. It's already fundamentally broken in the sense that alias
> this can make it incorrect if you're testing for inheritance, e.g.
> 
> ```
> void main()
> {
>      auto foo = new Foo;
>      A a = foo;
> 
>      static assert(is(Foo : A));
> }
> 
> class A {}
> 
> class Foo
> {
>      A a;
> 
>      alias this = a;
> }
> ```
> 
> Really, we need something in __traits to test for inheritance and to stop
> using is expressions for it.
> 
> Either way, it's fundamentally wrong to have is(Child : Parent) tell you
> that a struct named Child inherits from a struct named Parent, because they
> wouldn't have an implicit conversion, and : is testing specifically for an
> implicit conversion.
> 
> - Jonathan M Davis

Problem is, template parameters also should work, using ``Child : 
Parent`` syntax.

Otherwise the usability on this is going to be absolutely atrocious in 
meta-programming.