[Modules] Add 'no_undeclared_includes' module map attribute
The 'no_undeclared_includes' attribute should be used in a module to
tell that only non-modular headers and headers from used modules are
accepted.
The main motivation behind this is to prevent dep cycles between system
libraries (such as darwin) and libc++.
Patch by Richard Smith!
llvm-svn: 284797
diff --git a/clang/docs/Modules.rst b/clang/docs/Modules.rst
index ad71a95..e252c63 100644
--- a/clang/docs/Modules.rst
+++ b/clang/docs/Modules.rst
@@ -365,6 +365,8 @@
The ``extern_c`` attribute specifies that the module contains C code that can be used from within C++. When such a module is built for use in C++ code, all of the module's headers will be treated as if they were contained within an implicit ``extern "C"`` block. An import for a module with this attribute can appear within an ``extern "C"`` block. No other restrictions are lifted, however: the module currently cannot be imported within an ``extern "C"`` block in a namespace.
+The ``no_undeclared_includes`` attribute specifies that the module can only reach non-modular headers and headers from used modules. Since some headers could be present in more than one search path and map to different modules in each path, this mechanism helps clang to find the right header, i.e., prefer the one for the current module or in a submodule instead of the first usual match in the search paths.
+
Modules can have a number of different kinds of members, each of which is described below:
.. parsed-literal::