[PATCH] D120540: [Driver] Enable to use C++20 modules standalone by -fcxx-modules

[Chuanqi Xu via Phabricator via cfe-commits]

ChuanqiXu added a comment.

In D120540#3359394 <https://reviews.llvm.org/D120540#3359394>, @iains wrote:

> 1. I agree 100% that the driver needs to be able to process in "c++20 modules mode";

Yeah, not all the projects could be able to run in c++20 mode. We use `-std=c++17` + `-fcoroutines-ts` for C++20 coroutine before.

> there is some handling of sources that should be changed accordingly.

If I don't misunderstand, I guess we don't. Since all the code about C++20 modules are controlled by `CPlusPlusModules` variable. I think there wouldn't be codes controlled by `Cpp20` variables.

> 2. I believe that it is a general objective of the tooling folks (roughly SG15 participants) that C++20 modules should (eventually) be considered automatic for C++20+

If I understand correctly, clang would enable C++20 modules by default if C++20 is enabled.

> 3. There is at least one request from tooling folks that there should be an option to disable modules (even when in C++20  mode); this is a practical measure to avoid the case that it is impossible to build a TU because of some potential modules-related bug ...

I think `-fno-cxxmodules` could be the option now.

> 4. IMO it becomes increasingly important to try and decouple the clang modules from C++20 modules as much as possible.

100% agree. I think it should be beneficial to decouple them from command line, implementation and comments. (Now many comments are not precise. For example, when we talk about a `Module`. we are referring a module unit indeed. There are other examples.)

> So .. I was going to suggest that we might introduce -fmodules= {none, clang, c++20, ...} 
> with defaults picked:
>
>    fmodules => clang (i.e. the current meaning)
>   
>   !fmodules && C++20+ => c++20 (i.e. the objective of (2) above
>   Where there are other flags that imply C++20 that can switch c++20 mode as well
>   
>   otherwise the default would be "none"
>
> .. this provides for (3) ... since -std=c++20 -fmodules=none could be used.
>
> I do not have a patch for this proposal as of this time (my current patches assume (2) but do not meet the objective of (3))

The proposal is good to me though. I sent a patch (https://reviews.llvm.org/D113391) before to forbid the mixed use of clang modules and c++20 modules since it might be confusing if we use the combination `-fmodules -std=c++20`. But the comment shows that the current users of Clang Modules (mainly Google and Apple) wish a smooth switch from clang module to c++20 module. So I think the proposal which would break the current use cases would be not easy to land. 
(This patch wouldn't break any use case I think).


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