Allow default constructors for structs

[Dukc]

monkyyy kirjoitti 30.8.2024 klo 14.11:
> On Thursday, 29 August 2024 at 14:28:09 UTC, Ogi wrote:
>> struct T {
>>     S s;
>>     // implicit `@disable this();`
>> }
> 
> why? I *just* got bitten by nested structs being hard to init (with 
> terrible error messages), why not just fix the problem?

There is a reason. D is designed so that every type, except `noreturn`, 
has an `.init` value. It's supposed to be a value, not a function. This 
is why a struct cannot have a default constructor. If we break that 
pattern, then we run into some pretty strange situtions. For example, 
what would be the `.init` value of a static array of structs with a 
default constructor? What if the `.init` value of such a struct is used 
at CTFE? If the struct is a member of another struct, should the default 
constructor be called only once to determine `.init` value of the parent 
struct, or every time the parent struct is initialised?

However, I agree that structs (and unions) with context pointers are 
really annoying because they break that pattern of `.init` value for 
every type. Maybe it should be allowed to initialise nested structs with 
a null context.