// translation unit #1

export module M:C;

// translation unit #2

export module M:B;

import :C;

// translation unit #3

export module M;

export import :B;

// translation unit #4

import M;

First, the module-import-declaration in #3 imports partition module `M:B`. 

According to the following rule

Additionally, when a module-import-declaration in a module unit of some module M imports another module unit U of M, it also imports all translation units imported by non-exported module-import-declarations in the module unit purview of U.

There is a non-exported module-import-declaration in TU #2, and the module-import-declaration in #3 that imports `M:B` is in the module unit `#3` of module `M`, hence the module-import-declaration in #3 not only imports `M:B` but also imports `M:C`.  

[module.import] p7 also says  

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.  

The exported module-import-declaration at `#3` imports `M:B` and `M:C` as aforementioned, so `M:B` and `M:C` are exported by `#3`?  

GCC does not accept this interpretation, which only considers that `M:B` is exported by `#3`. Which are these TUs referred to by "such translation units" in "such translation units are said to be exported by T."? According to the context, "such translations units" should refer to all TUs that are imported by the module-import-declaration. Why does GCC only consider `M:B` as being exported?