It seems users are confused by the lack of standard library header
units.
gcc/
* doc/invoke.texi (C++ Modules): Document lack of std
library header units.
--
Nathan Sidwell
diff --git i/gcc/doc/invoke.texi w/gcc/doc/invoke.texi
index 054b8371593..8766bcdfc18 100644
--- i/gcc/doc/invoke.texi
+++ w/gcc/doc/invoke.texi
@@ -32891,6 +32891,13 @@ the purview of a named module. This is not implemented. Such
declarations will be attached to the module, if any, in which they are
declared.
+@item Standard Library Header Units
+The Standard Library is not provided as importable header units. If
+you want to import such units, you must explicitly build them first.
+If you do not do this with care, you may have multiple declarations,
+which the module machinery must merge---compiler resource usage can be
+affected by how you partition header files into header units.
+
@end table
Modular compilation is @emph{not} enabled with just the
