I don't understand what you're saying here. #pragma once does the job that include guards used to do, but with less work, and in a less error prone way. How is it broken, and how is the size of a project relevant?

> Does anyone write those by hand anyway in any kind of project the size where it would matter?

I think you're suggesting that you don't need to make up the names for include guards because all tools / IDEs for C++ write them for you automatically anyway. But that isn't my experience. Many IDEs don't write include guards for you automatically ... because everybody uses #pragma once already.

> #pragma once is broken by design

I think you're referring to the historical problem with #pragma once, which is that it can be hard for the compiler to identify what is really the same file (and therefore shouldn't be included a second time). If you hard link to the same file, or soft link to it, is it the same? What if the same file is mapped to two different mount points? What if genuinely different files have the same contents (e.g., because the same library is included from two different installation paths)? In practice, soft/hard links to the same file are easily detectable, and anything more obscure indicates such a weird problem with your setup that you surely have bigger issues. #pragma once is fine.

(Historically, it also had the benefit that compilers would know not to even re-read the header, whereas with traditional include guards they would need to re-include the file (e.g. in case the whole file is not wrapped in the #ifdef, or in case something else has undefined it since) only to then discard the contents. I've even seen coding guidelines requiring external include guards wrapped around every use of headers with #include <...>. Yuck! But modern compilers can work out when include guards are meant to mean that so today that difference probably no longer exists.)

Even if you don't write header guards by hand you get issues. The amount of time I got bitten by someone naming a file "widget.h" or "utils.hpp" three levels of libraries down with the corresponding #ifndef WIDGET_H which broke the build in incredibly mysterious ways...

https://github.com/search?q=ifndef+WIDGET_H&type=code

What major compiler does not support it?

Yes, it is non-standard, but I don't know any compiler that does not support it.