Guard includes of standard library headers so users can use import std

[davidhunter22]

fixes #1217

This is an example of how issue #1217 could be fixed

[carsonRadtke]

Thanks for the change! Adding modules to a project is new to me, so it will take a little bit of time for me to learn the things necessary to properly review this PR.

In the meantime, can you add a cmake variable that controls GSL_BUILD_USING_STD_MODULE as well as a test case to verify your change behaves as expected?

[davidhunter22]

I will add can you add a cmake variable that controls GSL_BUILD_USING_STD_MODULE and a test, I was just very wary about changing CMake stuff.

I can do this in two ways.

1. Add a CMake option. Add a single project using this option that builds using import std; and leave the rest alone. So a minimal set of changes.
2. Add a CMake option. Modify all existing test so the work with import std;'. This is I think all macro guarding #include` of standard library headers.

Let me know which you would prefer.
I would suggest I submit this as a separate PR unless you want all the changes together.

[davidhunter22]

I did notice one issue with using GoogleTest. On MSVC it compiles with Unicode defined and with the "/J" and "UTF-8" flags set. The GSL tests compile with MultiByteCharacterSet and do not set "/J" and "UTF-8". I'm not sure this is compatible. It certainly complains about it when I use import std; which is more picky about these sorts of things.

[carsonRadtke]

I can do this in two ways.
...
I would suggest I submit this as a separate PR unless you want all the changes together.

Feel free to submit the cmake option change as a separate PR. I don't think every test needs to be converted immediately, but I'd like to at least see one test in this PR that has GSL_BUILD_USING_STD_MODULE defined (whether through cmake, through the build command line or via a source definition). This can be a new test, or a converted existing test.

I did notice one issue with using GoogleTest.

Please open a separate issue for this.

[davidhunter22]

On the "/J" and "UTF-8" flags I'm not sure I'm qualified to make an opinion.
Is it Ok to mix them, I'm not sure. All I do know is when I tried building using import std; if was getting warning about incompatible flags.
Given that GSL is a MicroSoft supported thing do you have an in with the compiler devs to ask them if mixing these flags is OK?

[davidhunter22]

I will create a PR for the test
I will also create a PR with the changes necessary to get all the test working so you can take a look at it.

[carsonRadtke]

Given that GSL is a MicroSoft supported thing do you have an in with the compiler devs to ask them if mixing these flags is OK?

Yes, I'm happy to link with compiler team to get clarification on this, I would just like a separate issue tracking the task (preferably with the diagnostic you get when mixing these flags when using modules).

Edit: Basically, this should either be an issue reported in GSL or an issue reported to the Developer Community, let's start with GSL and I can escalate it if necessary.

[davidhunter22]

I am still on this. I am hoping the #1222 and CmakePresets.json changes land soon as that will make life a bit easier