What are (were) the most difficult parts of D?

[Siemargl]

On Wednesday, 11 May 2022 at 05:41:35 UTC, Ali Çehreli wrote:
> What are you stuck at? What was the most difficult features to 
> understand? etc.
>
> To make it more meaningful, what is your experience with other 
> languages?
>
> Ali

When i take old library from a dub or github and cannot compile 
it with latest compiler because of small and subtle language 
changes.

For example
https://github.com/buggins/dlangide/pull/410/commits/43a82ed2f45143270482aa94c447e577174f0108#diff-375ecd44ef501b93adc6106fe404f55d353e667c21681c1c13a9e2008510ab42L937

I dont know even now, which D's features changes here.