[module.private.frag]

A private-module-fragment shall appear only in a primary module interface unit ([module.unit]).

A module unit with a private-module-fragment shall be the only module unit of its module; no diagnostic is required.

[Note 1:

A private-module-fragment ends the portion of the module interface unit that can affect the behavior of other translation units.

A private-module-fragment allows a module to be represented as a single translation unit without making all of the contents of the module reachable to importers.

The presence of a private-module-fragment affects:

(2.1)
the point by which the definition of an inline function or variable is required ([dcl.inline]),
(2.2)
the point by which the definition of an exported function with a placeholder return type is required ([dcl.spec.auto]),
(2.3)
whether a declaration is required not to be an exposure ([basic.link]),
(2.4)
where definitions for inline functions and templates must appear ([basic.def.odr], [dcl.inline], [temp.pre]),
(2.5)
the instantiation contexts of templates instantiated before it ([module.context]), and
(2.6)
the reachability of declarations within it ([module.reach]).

— end note]

[Example 1: export module A; export inline void fn_e(); / error: exported inline function fn_e not defined / before private module fragment inline void fn_m(); / error: non-exported inline function fn_m not defined static void fn_s(); export struct X; export void g(X *x) { fn_s(); / OK, call to static function in same translation unit } export X *factory(); / OK module :private; struct X {}; / definition not reachable from importers of A X *factory() { return new X (); } void fn_e() {} void fn_m() {} void fn_s() {} — end example]

10 Modules [module]

10.5 Private module fragment [module.private.frag]