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" ||
----------------
philnik wrote:
> aaron.ballman wrote:
> > philnik wrote:
> > > 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.
> > > 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?
> >
> > Why would the library want a diagnostic telling them they're using a
> > reserved identifier as a module name?
> >
> > > 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.
> >
> > It also allows users to produce modules with reserved identifiers. It's an
> > error that can't be downgraded specifically because I don't think we want
> > our implementation to give arbitrary users that ability.
> > > 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?
> >
> > Why would the library want a diagnostic telling them they're using a
> > reserved identifier as a module name?
>
> I don't mean specifically this error, I mean more generally that other
> warnings should be generated from std modules. Treating the headers as system
> headers disables most warnings, which is the reason libc++ treat them as
> normal headers in the tests.
>
> > > 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.
> >
> > It also allows users to produce modules with reserved identifiers. It's an
> > error that can't be downgraded specifically because I don't think we want
> > our implementation to give arbitrary users that ability.
>
> I think there should be some way to enable normal warnings in the special
> modules, since it makes the life of library developers a lot easier. I don't
> care whether that's through disabling a warning or some special sauce to
> enable warnings from the std module, but there should be some way.
>
> I don't mean specifically this error, I mean more generally that other
> warnings should be generated from std modules. Treating the headers as system
> headers disables most warnings, which is the reason libc++ treat them as
> normal headers in the tests.
Ahhh, thank you, that makes a lot more sense to me. :-D
> I think there should be some way to enable normal warnings in the special
> modules, since it makes the life of library developers a lot easier. I don't
> care whether that's through disabling a warning or some special sauce to
> enable warnings from the std module, but there should be some way.
Just like we have `-Wsystem-headers`, I would expect we'd have something
similar for modules (or reuse it, perhaps with a different name, for both
headers and modules).
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