philnik 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" ||
----------------
ChuanqiXu wrote:
> aaron.ballman wrote:
> > aaron.ballman wrote:
> > > ChuanqiXu wrote:
> > > > erichkeane wrote:
> > > > > ChuanqiXu wrote:
> > > > > > aaron.ballman wrote:
> > > > > > > ChuanqiXu wrote:
> > > > > > > > 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.
> > > > > > > > then we need to build the std modules in our working directory,
> > > > > > > > which is not system directories.
> > > > > > >
> > > > > > > `-isystem`, pragmas, and linemarkers are all ways around that --
> > > > > > > I don't think we need a feature flag for this, unless I'm
> > > > > > > misunderstanding something.
> > > > > > Although it may be a little bit nit picker, the module unit which
> > > > > > declares the std modules won't be header. It is a module unit. So
> > > > > > it requires we implement std modules by wrapping linemarkers around
> > > > > > the std modules declaration, which looks a little bit overkill.
> > > > > >
> > > > > > And another point is that maybe we need to introduce another
> > > > > > feature flags to implement std modules. Although I tried to
> > > > > > implement std modules within the current features, I can't prove we
> > > > > > can implement std modules in that way in the end of the day.
> > > > > >
> > > > > > Let me add some more words. The standards require us to implement
> > > > > > std modules without deprecating the system headers. This strategy
> > > > > > leads the direction to "implement the components in the original
> > > > > > headers and control their visibility in the std module unit".
> > > > > > It may look like:
> > > > > >
> > > > > > ```
> > > > > > //--- std.cppm
> > > > > > module;
> > > > > > #include <algorithm>
> > > > > > ...
> > > > > > export module std;
> > > > > > ```
> > > > > >
> > > > > > Then how can control the visibility? In my original experiments, I
> > > > > > use the style:
> > > > > >
> > > > > > ```
> > > > > > //--- std.cppm
> > > > > > module;
> > > > > > #include <algorithm>
> > > > > > ...
> > > > > > export module std;
> > > > > > export namespace std {
> > > > > > using std::sort;
> > > > > > }
> > > > > > ```
> > > > > >
> > > > > > but this doesn't always work. For example, we can't `using` a
> > > > > > in-class friend definition. And there are more reasons, the
> > > > > > unreachable declarations in the global module fragment (the section
> > > > > > from `module;` to `export module [module_name]`) can be discarded
> > > > > > to reduce the size of the module file. And the reachable rules are
> > > > > > complex. But the simple story is that it is highly possible the a
> > > > > > lot of necessary declarations in global module fragment in the
> > > > > > above snippet would be discarded so that the user can't use std
> > > > > > modules correctly. I mean, we **may** need a special feature flag
> > > > > > for it. And the method with `system headers` looks not good and
> > > > > > semantics are not so consistency.
> > > > > >
> > > > > >
> > > > > IMO, any such additional flag (say `-isystem-module`) should ALSO use
> > > > > the `isInSystemHeader` infrastructure. I suspect nearly every place
> > > > > we use `isInSystemHeader` we also mean to exclude a system-module as
> > > > > well.
> > > > >
> > > > > I think that any such flag can/should be added later as you figure
> > > > > out how it should be specified/work. That said, when you do so, it
> > > > > should either also feed `isInSystemHeader`, or basically every use of
> > > > > `isInSystemHeader` should ALSO changed to use the new flag as well
> > > > The main confusion part to me is that why we connect `std modules` with
> > > > system paths? I know implementors can work around the check like the
> > > > tests did. But what's the point? I know every header of libcxx
> > > > contains:
> > > >
> > > > ```
> > > > #ifndef _LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER
> > > > # pragma GCC system_header
> > > > #endif
> > > > ```
> > > >
> > > > but it is for the compatibility with GCC. And it looks not so
> > > > meaningful to force the implementation of modules to keep such
> > > > contraints.
> > > > I think that any such flag can/should be added later as you figure out
> > > > how it should be specified/work. That said, when you do so, it should
> > > > either also feed isInSystemHeader, or basically every use of
> > > > isInSystemHeader should ALSO changed to use the new flag as well
> > >
> > > +1, that's my thinking as well.
> > > The main confusion part to me is that why we connect std modules with
> > > system paths?
> >
> > We consider the system paths to be "special" in that they can do things
> > "user" paths cannot do. I think we want to keep that model for modules
> > because of how successful it has been for includes. (e.g., don't suggest
> > fixits in a system module but do suggest them for user modules).
> OK, I got it and it won't be a problem we can't workaround.
IIUC this would prevent the library from handling the `std` module the same as
a user module, right? AFAIK the actual use of
`_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER` is to enable warnings in the headers for
development, which would not work with the modules with this patch, or am I
misunderstanding something? Is there a reason this isn't a warning that's an
error by default? That would allow the library to disable it and still serve
the same purpose.
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https://reviews.llvm.org/D136953/new/
https://reviews.llvm.org/D136953
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