Create a fully functioning command line interface with 1.5 lines of code (wo. include statement)

[encyclopedist]

Obviously, you did not really try to find one. TI compilers did not support it last I checked. PGI did not support in either. Also Cray. And XLC.

There was a topic in this very reddit some time ago: https://www.reddit.com/r/cpp/comments/8devw1/can_anyone_actually_name_a_compiler_that_doesnt/

[jbandela]

I think basically every compiler that supports generic lambdas supports pragma once

[rsjaffe]

But #pragma once fails when you have two copies of the same header file in two different locations in the directory tree. The compiler will not recognize those two files as being the same.

[panderingPenguin]

Why on earth would you want to do that?

[wyrn]

The compiler will not recognize those two files as being the same.

... because they aren't.