Standard library module

[davidhunter22]

The latest Visual Studio compiler 17.5 has standard library modules.

I am wondering if you want to support using the standard library via a module as well as old style includes.
I have a local branch where I can include all the type_safe header files in a test program using "import std;
If you are interested in this I can push a draft PR for you to look at. I tried to make the minimal amount of changes and to make sure that the default behaviour is everything works as is.

The main changes are

1. Use a macro in all the headers to choose whether to import or include. Obvisouly the default is to include. For example

#include <type_traits>

becomes

#if defined(TYPE_SAFE_IMPORT_STD_MODULE)
import std;
#else
#include <type_traits>
#endif

In theory you should be able to mix import and old style headers but the MS compiler does not support that yet and may not for a while see microsoft/STL#3195 for STLs comment about this. In any case I think it's a good idead to macro out the old stuff.
2) Fix a couple of cases where the headers assumed things like int8_t were in the global namespace. Note that this cou;d be fixed by using std.compat but I think it's good to not really on global namespace pollution
3) There appears to be a compiler bug that is trigger by including the type_safe header using /std:c++latest switch. I have a MS dev community issues filed for it https://developercommunity.visualstudio.com/t/VS-22-174-templated-constructor-paramet/10202157. I put a couple of fixes in the code guarded by macors so the exiting code is the default.
4) The test program is a new project and is off by default in CMake

option(TYPE_SAFE_IMPORT_STD_MODULE "Build using the standard library modules rather than #include" OFF)

if(TYPE_SAFE_IMPORT_STD_MODULE)
    add_subdirectory(test_std_module/)
endif()

I believe this includes all the changes needed to make the type_headers work with std module. A furhter step would be to get all the test programs and examples to build as well. I could do this as a follow up PR or include it in this one although that will obviously make the diff bigger.

Anyway let me know how you feel or if you have any suggestions.

[davidhunter22]

BTW including the type_safe headers with /std::latest does give a few warnings

'std::aligned_storage<4,4>': warning STL4034: std::aligned_storage and std::aligned_storage_t are deprecated in C++23. Prefer alignas(T) std::byte t_buff[sizeof(T)].

Also the code is at https://github.com/davidhunter22/type_safe/tree/std_module

[foonathan]

I'm happy for any and all PRs.

The main changes are [...]

Sounds good. Please split bugfixes and compiler workaround into separate PRs.

A furhter step would be to get all the test programs and examples to build as well. I could do this as a follow up PR or include it in this one although that will obviously make the diff bigger.

Let's focus on things affecting end users first.

BTW including the type_safe headers with /std::latest does give a few warnings

Right, these should be replaced by alignas + char array. Can you do a separate PR for that?