P1979R0: Resolution to US086

Author: Mathias Stearn
Date: 2019-11-08

Effect of this change

This change is intended to prevent import declarations in the global module fragment of a module unit from being implicitly imported when other units of the same module import that unit. Other NB comment resolutions will ensure that such import declarations must come from #include expansion (possibly by translation to an import) and must not be explictlity exported.

This is believed to be an unintentional change in behavior from the Modules TS caused by a change in specification mechanisms.

Example

// uses_vector.h
import <vector>; // Could be from a translated #include

// partition.cpp
module;
#include "uses_vector.h" // textually expands to import <vector>;
module A:partition;

// interface.cpp
module A;
import :partition;
std::vector<int> x;

This change makes the last line ill-formed because it refers to std::vector which is not visible. Previously the import of <vector> was implicitly imported into the interface.cpp translation unit.

Wording

Modify 10.3 [module.import] paragraph 6 as follows:

When a module-import-declaration imports a translation unit T, it also imports all translation units imported by exported module-import-declarations in T; such translation units are said to be exported by T. Additionally, wWhen a module-import-declaration in a module unit of some module M imports another module unit U of ~~the same module~~ M, it also imports all translation units imported by non-exported~~all~~ module-import-declarations in the module unit purview of U~~that module unit~~. [ Footnote: This is consistent with the rules for visibility of imported names ([basic.scope.namespace]). — end footnote ] These rules may in turn lead to the importation of yet more translation units.