aaron.ballman marked an inline comment as done.
aaron.ballman 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" ||
----------------
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).
================
Comment at: clang/test/Modules/reserved-names-1.cpp:33
+ expected-error {{module declaration must
occur at the start of the translation unit}}
+
+// Show that being in a system header doesn't save you from diagnostics about
----------------
ChuanqiXu wrote:
> We lack a test for `foo.std`;
reserved-named-2.cpp has that test (it uses `std0` instead of `std`). Is that
sufficient?
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D136953/new/
https://reviews.llvm.org/D136953
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