[PATCH] D136953: [C++20] Diagnose invalid and reserved module names

[Chuanqi Xu via Phabricator via cfe-commits]

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" ||
----------------
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.


================
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
----------------
We lack a test for `foo.std`;


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  https://reviews.llvm.org/D136953/new/

https://reviews.llvm.org/D136953