[cfe-dev] Modules, redeclaration chains, and locally-scoped extern declarations

[Richard Smith via cfe-dev]

On Thu, 26 Sep 2019 at 14:38, Jordan Rose via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi, modules folks and cfe-dev. I *think* I've discovered a bug in how
> module loading handles redeclaration merging, but it might also be by
> design. (And honestly "by design" would be nice in my particular use case.)
> Here it is:
>
> // ModuleA.h
> void test() {
>   extern void sharedFunc(void);
> }
>
> // ModuleB.h
> void sharedFunc(int);
>
> // client.c
> #import <ModuleA.h>
> #import <ModuleB.h>
> // or the other order, for a slightly different experiment
>
>
> In this situation, the locally-scoped declaration of 'sharedFunc' in
> ModuleA isn't resolved with the declaration in ModuleB. There's even a
> conflict in their types! Is this by design, or an oversight, or what?
>

Well.. we only merge declarations that have matching types. This decision
predates my involvement, but I think it makes some sense: we probably don't
want the deserialization code somewhat nondeterministically producing
diagnostics when it notices problems like this, and deferring the
diagnostic to an ambiguity error when the function is actually used seems
like the most consistent user experience. And that ambiguity error will
presumably not appear for a call to sharedFunc(), because
(non-redeclaration) lookup for sharedFunc won't find the local extern
declaration in 'test'. (But I would not be surprised to see an error if you
attempt to redeclare sharedFunc after importing both modules, or if you try
to build both headers as part of the same module.)

Regardless of whether we accept it, the above program is ill-formed
(undefined in C / ill-formed with no diagnostic required in C++), so I
would encourage you to limit your reliance on it. I don't think we want to
guarantee that such code keeps compiling.

One of the consequences of this is that in client.c's context, asking for
> the canonical decl for 'sharedFunc' gives me the declaration in ModuleB.
> From client.c's perspective, that's *more* useful than giving me the one
> in ModuleA, but it's not consistent with the behavior with modules turned
> off.
>
> Any insights to offer? Should I file this at bugs.llvm.org?
>

We generally don't produce eager diagnostics for conflicts between imported
modules, and that's a feature: we want module import to be constant-time,
which means we can't diagnose such things eagerly. So for the above program
(with no use of sharedFunc), I don't think we should produce a diagnostic.

I think the current behavior is largely by design.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20190926/f0cf802b/attachment.html>