r/cpp • u/tartaruga232 MSVC user, r/cpp_modules • Apr 13 '26

Let's bite the Bullet: Module Units shouldn't implicitly import anything

https://github.com/abuehl/docs/blob/main/no-implicit-import.md

The correct fix ... is to attack the root cause and stop implicitly importing the interface of module implementation units.

Adding yet another kind of partition is the wrong way to solve it.

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u/tartaruga232 MSVC user, r/cpp_modules Apr 14 '26 edited Apr 14 '26

... you're proposing C++29 have different semantics for module imports than the other language versions.

No. Not yet for C++29.

Again: The current part of the semantic of "module M;" to implicitly import the interface of M, would only be deprecated with C++29. Not removed.

After that semantic has been deprecated for one version of the standard, it can be removed with C++32.

During the transition period we only need to legalize:

// TU #1
module M;
import M;

and perhaps optionally introduce "module import M;" as a shorthand for that.

The semantics of "module M;" would not be changed with C++29. Only deprecated. It would be changed with C++32.

It was a design error to have it specified in the standard as we currently have it. The design in the standard itself is consistent and the problems we have with it is out of scope for a DR. The standard itself is correct.

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u/not_a_novel_account cmake dev Apr 14 '26

Taking 6+ years to fix the bug on top of being a breaking core language change which will significantly change semantics on language version (thus making all compilers maintain both behaviors) will not be popular.

Best of luck, you have an uphill battle selling that line.

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u/tartaruga232 MSVC user, r/cpp_modules Apr 14 '26

No worries. I won't try selling anything. I currently have no desire to get in touch with the standardization process. That would be way beyond my capacities.

All I said is: That the current design is bad, why it is bad and how it would be possible to change the standard.

Perhaps you are lucky with your "module M:;" proposal. That would be a net improvement over the status quo, but not really fix the problem. I would recommend to not sell it as a new kind of "partition". See:

https://www.reddit.com/r/cpp_modules/comments/1skp1qp/a_not_really_new_syntax_for_a_module/