Let's get the semantic around closure fixed.

[Paul Backus]

On Wednesday, 19 May 2021 at 19:01:59 UTC, Walter Bright wrote:
> Languages like D also need to be useful, not just correct. 
> Having a hidden allocation per loop will be completely 
> unexpected for such a simple looking loop for a lot of people. 
> That includes pretty much all of *us*, too.

If closures causing "hidden" allocations is problematic, from a 
language-design perspective, then it's problematic whether it 
occurs inside a loop or not. Either we should (a) deprecate and 
remove GC-allocated closures entirely, or (b) make them work 
correctly in all cases.

> It's best to just return a compile error for such cases rather 
> than go to very expensive efforts to make every combination of 
> features work.

This is the worst of both worlds: we still pay the price of 
having "hidden" allocations in our code, but we do not even get 
the benefit of having properly-implemented closures in return.