[doc] [Modules] Document how to implement ABI compatible code in module units

diff --git a/clang/docs/StandardCPlusPlusModules.rst b/clang/docs/StandardCPlusPlusModules.rst
index bcf2c4470c5213dfcdc73478480c44a1e37b1026..970803b56c8c6e95d0669a21fbe316640f4d23aa 100644 (file)
--- a/clang/docs/StandardCPlusPlusModules.rst
+++ b/clang/docs/StandardCPlusPlusModules.rst
@@ -591,6 +591,19 @@ The result would be ``NS::foo@M()``, which reads as ``NS::foo()`` in module ``M`
 The ABI implies that we can't declare something in a module unit and define it in a non-module unit (or vice-versa),
 as this would result in linking errors.
 
+If we still want to implement declarations within the compatible ABI in module unit,
+we can use the language-linkage specifier. Since the declarations in the language-linkage specifier
+is attached to the global module fragments. For example:
+
+.. code-block:: c++
+
+  export module M;
+  namespace NS {
+    export extern "C++" int foo();
+  }
+
+Now the linkage name of ``NS::foo()`` will be ``_ZN2NS3fooEv``.
+
 Known Problems
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