The C++ reference gives an example of a module partition implementation unit (A-C.cpp) being used without having to define a interface unit alongside it:

/////// A-B.cpp   
export module A:B;
...
 
/////// A-C.cpp
module A:C;
...
 
/////// A.cpp
export module A;
 
import :C;
export import :B;
 
...

However, in my code – compiled with MSVC – the first example works, but the second doesn’t:

/////// game.ixx
export module game;

import :rendering;

...

/////// rendering.ixx
export module game:rendering;  // this works

...

vs

/////// game.ixx
export module game;

import :rendering;

...

/////// rendering.cpp
module game:rendering;  // this doesn't work

...

From what I understand, removing the export declaration in rendering.ixx (and changing the extension back to .cpp for MSVC, but I tried both) turns that partition into a implementation unit, which is something I want to do because I want to keep my rendering functions internal. However, I get a “module partition 'rendering' for module unit 'game' was not found” error when I do that.

(Also, this is tangential but why doesn’t game.ixx throw an error because I didn’t export-import the :rendering partition in game.ixx? The C++ reference says:)

Module partitions can be module interface units (when their module declarations have export). They must be export-imported by the primary module interface unit, and their exported statements will be visible when the module is imported.