[PATCH] D113391: [Modules] Don't support clang module and c++20 module at the same time

[Jan Svoboda via Phabricator via cfe-commits]

jansvoboda11 added inline comments.


================
Comment at: clang/test/ClangScanDeps/Inputs/header-search-pruning/cdb.json:3
+  "directory" : "DIR",
+  "command" : "clang -E DIR/header-search-pruning.cpp -Ibegin -I1 -Ia -I3 -I4 -I5 -I6 -Ib -I8 -Iend DEFINES -fmodules -fmodules-cache-path=DIR/module-cache -fimplicit-modules -fmodule-map-file=DIR/module.modulemap",
+  "file" : "DIR/header-search-pruning.cpp"
----------------
ChuanqiXu wrote:
> jansvoboda11 wrote:
> > ChuanqiXu wrote:
> > > jansvoboda11 wrote:
> > > > Can you explain why `-fcxx-modules` is removed? We want to explicitly enable Clang modules for C++ inputs here. That's off by default in our downstream repo and we'd like to keep this upstream to prevent conflicts. (it's benign upstream)
> > > According to the discussion in the link, I think it is in consensus that we decide to not support clang modules and c++20 modules at the same time. (I know many people may not have visited it. If any one disagree with the higher idea, I think it would be better to reply in that thread). 
> > > So the original test case would fail.
> > > 
> > > > We want to explicitly enable Clang modules for C++ inputs here. 
> > > 
> > > I am not sure if I missed any thing. Personally, it looks like the test now would test clang modules for c++ inputs. Since it has `-fmodule` option so that clang module is enabled and the input is C++ (from its suffix). Do I misunderstand you?
> > > According to the discussion in the link, I think it is in consensus that we decide to not support clang modules and c++20 modules at the same time. (I know many people may not have visited it. If any one disagree with the higher idea, I think it would be better to reply in that thread). 
> > > So the original test case would fail.
> > 
> > What's the error message? As I say, we're enabling Clang modules for C++ input here, not C++20 modules.
> > 
> > > > We want to explicitly enable Clang modules for C++ inputs here. 
> > > 
> > > I am not sure if I missed any thing. Personally, it looks like the test now would test clang modules for c++ inputs. Since it has `-fmodule` option so that clang module is enabled and the input is C++ (from its suffix). Do I misunderstand you?
> > 
> > Right. What I'm saying is that in our downstream repo, `-fmodules` is not enough to enable Clang modules for C++ inputs, you need to do it via `-fcxx-modules`. And we'd like to keep it upstream, even though it's benign here, to avoid conflicts between the repos.
> > What's the error message? As I say, we're enabling Clang modules for C++ input here, not C++20 modules.
> 
> The error message is added in this patch. After this patch landed, the original intentional behavior would be `-fmodules` to enable clang module and `-fcxx-modules` to enable C++20 modules.
> 
> > Right. What I'm saying is that in our downstream repo, -fmodules is not enough to enable Clang modules for C++ inputs, you need to do it via -fcxx-modules. And we'd like to keep it upstream, even though it's benign here, to avoid conflicts between the repos.
> 
> Got it. But it conflicts with the idea that disable clang module and c++20 module at the same time. Personally, I think it would be better to edit the code in downstream. @aaron.ballman sorry if I ping you too frequently. But I think now we need input from you.
Ah, I understand now, thanks for explaining.

I'm worried about changing the behavior of a driver flag, we generally don't break existing driver options. Have you considered keeping the `-fmodules` and `-fcxx-modules` semantics intact and instead add new `-fno-c++20-modules` flag or something like that?


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  https://reviews.llvm.org/D113391/new/

https://reviews.llvm.org/D113391