[PATCH] D122119: [C++20][Modules] Adjust handling of exports of namespaces and using-decls.

[Chuanqi Xu via Phabricator via cfe-commits]

ChuanqiXu added a comment.

In D122119#3398823 <https://reviews.llvm.org/D122119#3398823>, @iains wrote:

> So the adjustment to the error message is something I am 50/50 about (IMO it makes some messages more useful, but maybe not needed in others).
>
> Without the change we get 
> "cannot export redeclaration 'xxx' here since the previous declaration is not exported"
>
> So, e.g in C++20 10.2 example 6. every case has the same error message (which was what prompted me to make the change).
>
> With the change here we now get:
> cannot export redeclaration 'f' here since the previous declaration has internal linkage
> cannot export redeclaration 'S' here since the previous declaration has module linkage
>
> which seems maybe to be more helpful to the user in telling them why.
>
> I hope others can weigh in with an opinion here  .. @dblaikie @vsapsai  ?

I am OK to wait for opinions from others. Let me talk it a little bit more.

The first feeling I saw the change is that not every C++ programmer knows about linkage. OK, it depends on the environment really and every one might has their own opinion.

Another thought is that 10.2.6 (http://eel.is/c++draft/module.interface#6) doesn't talk anything about linkage:

> A redeclaration of an entity X is implicitly exported if X was introduced by an exported declaration; otherwise it shall not be exported.

So it looks like confusing to talk about linkage this time. In my imagination, there might be a such situation:

A programmer met the error when he tries to export a redeclaration which is internal linkage (maybe a simple const variable). Then the message told him the internal linkage is not allowed to re-export. Then he removes the const specifier. Now he meets the error again. It tells that we couldn't export redeclaration which is module linkage. I guess he would feel bad. Then he might try to export the first declaration to get passed. However, the `const` specifier is lost in the case. And in the current message, I guess he would add export to the first declaration directly after he reads the message.


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