It is the year 2020: why should I use / learn D?

[Guillaume Piolat]

On Wednesday, 14 November 2018 at 15:07:46 UTC, lagfra wrote:
> https://www.reddit.com/r/cpp/comments/9vwvbz/2018_san_diego_iso_c_committee_trip_report_ranges/
>
> By 2020 C++ is planning to introduce:
>
> * Ranges
> * Contracts
> * Concepts (`__traits`)
> * Proper constexpr
> * Modules
> * Reflections
> * Green threads
>

Not all implementations are equal.

- Regarding `constexpr`

For example D's CTFE works by having some places where things are 
forced to be evaluated in CTFE. No syntax.
In C++, you currently have `constexpr` and soon you'll have 
`consteval` alongside with constexpr 
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1073r2.html). No new keyword was used for the D version of CTFE.


Similarly, `if constexpr` is strictly less useful than `static 
if` since it introduces a scope. See the workaround here: 
https://stackoverflow.com/questions/46880578/scope-of-variables-declared-inside-if-constexpr-blocks < back to where we were with enable_if in terms of code size.


Regarding modules, they were proposed and reported in two C++ 
standards already.
Same for concepts.


Ultimately all this doesn't matter much since:
- the C++ culture of using cmake/makefiles over a declarative 
build system will not change (see the demise of biicode)
- avoiding the latest language addition to keep compatibility is 
necessary in C++, with divergent frontends
- existing codebases are still C++98 for the most part

D has the chance to have a community :
- for which template meta-programming is simple, well understood 
and almost boring
- which is willing to build an ecosystem (but not enough)

>
> what are the advantages of using D vs C++ if my goal is to 
> build a complex system / product?

Easy: your struggle to learn the D language will have an ending. 
Which lead to reduced mental load forever - without compromising 
anything in power.


> TL;DR: what will D offer with respect to C++ when almost all 
> key features of D are present in C++20(+)?

Sanity.