Allow this() { } for structs

[FeepingCreature]

On Monday, 2 November 2020 at 19:53:31 UTC, Q. Schroll wrote:
> Can we just allow this() when the body is essentially empty 
> (that is: after rewrites and lowerings contains no statements)? 
> The compiler would happily accept the code and pretend this() { 
> } isn't there. It would even reduce confusion because S() is 
> legal as an expression, but this() isn't as a constructor. With 
> that, this() is legal as a constructor, but it must not do 
> anything.

I just want this() always, even with a non-empty body.

We run into an issue where we need predictable destructor calls 
for a struct constructed for a with(), but we also want possibly 
immutable data in the struct, so we *have* to use this() rather 
than static opCall() which is the standard workaround.

So we end up with code like this:

struct StructType {
   this(int) { ... }
}

StructType structType() { return StructType(0); }

I do not understand what purpose the lack of this() serves. I 
mean, I understand what purpose it serves, it's so T.init is 
always equivalent to T(), but I don't understand what purpose 
*that* serves. T.init is *already* not a valid constructed T if 
you want to use any invariants and/or call any methods at all. We 
*already* can't treat them equivalently regardless, so from my 
perspective the ban on T() serves no purpose.