ChuanqiXu added inline comments.

================
Comment at: clang/lib/Sema/SemaModule.cpp:282
+  StringRef FirstComponentName = Path[0].first->getName();
+  if (!getSourceManager().isInSystemHeader(Path[0].second) &&
+      (FirstComponentName == "std" ||
----------------
cor3ntin wrote:
> ChuanqiXu wrote:
> > std modules should be irreverent with system headers; The intuition of the 
> > wording should be that the users can't declare modules like `std` or 
> > `std.compat` to avoid possible conflicting. The approach I imaged may be 
> > add a new compilation flags (call it `-fstd-modules`) now. And if the 
> > compiler found a `std` module declaration without `-fstd-modules`, emit an 
> > error.  
> > 
> > For now, I think we can skip the check for `-fstd-modules` and add it back 
> > when we starts to support std modules actually.
> The idea is that standard modules are built from system directories... it 
> seems a better heuristic than adding a flag for the purpose of 1 diagnostics 
> ( maybe some other system library could in theory export std with no warning, 
> but I'm not super worried about that being a concern in practice)
> The idea is that standard modules are built from system directories...

This is not true. For example, if someday libc++ supports std modules, then we 
need to build the std modules in our working directory, which is not system 
directories. And **ideally**, we would distribute and install module file in 
the system directories. But it is irreverent with the path of the source file.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D136953/new/

https://reviews.llvm.org/D136953

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