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