header include cycle problematic for modules

[urnathan]

There is a cycle between container_hash/hash.hpp and container_hash/extensions.hpp. This is problematic for modules (both clang and C++ header units). This cycle is made acceptable by removing extensions.hpp's idempotency and have it pre-include hash.hpp if that's not already been included. Then, should hash.hpp include extensions.hpp the later conditional will nullify the (second) tokenization of extensions.hpp's body. This preserves the existing textual-header behaviour of the two files. But, it also makes them buildable as clang-headers, because the cycality remaining in the graph is unproblematic.

An alternative would be to move the body of extensions.hpp to extensions-impl.hpp, and have hash.hpp conditionally include that. Then turn extensions.hpp into a stub that include hash.hpp with BOOST_HASH_NO_EXTENSIONS undefined so it includes extensions-impl. that way extensions-impl.hpp and extensions.hpp can be idempotent (but hash.hpp has to remain not)

+++ boost/container_hash/extensions.hpp
@@ -10,15 +10,22 @@
 // This implements the extensions to the standard.
 // It's undocumented, so you shouldn't use it....

+#if !defined(BOOST_FUNCTIONAL_HASH_HASH_HPP)
+// container_hash/hash.hpp and container_hash/extensions.hpp
+// (conditionally) include each other. This causes problems for clang
+// modules (and c++ header units). So, we include
+// container_hash/hash.hpp *outside our header protection* if it's not
+// already included. That in turn might reinclude us, in which case
+// when we get back to here, our header protection will kick in. When
+// building as a module, the cycle is unproblematic.
+#include <boost/container_hash/hash.hpp>
+#endif
+
 #if !defined(BOOST_FUNCTIONAL_HASH_EXTENSIONS_HPP)
 #define BOOST_FUNCTIONAL_HASH_EXTENSIONS_HPP

 #include <boost/config.hpp>
-#if defined(BOOST_HAS_PRAGMA_ONCE)
-#pragma once
-#endif

-#include <boost/container_hash/hash.hpp>
 #include <boost/detail/container_fwd.hpp>
 #include <boost/core/enable_if.hpp>
 #include <boost/static_assert.hpp>

[pdimov]

This will be taken care of when I merge the develop branch, but it's not ready for prime time yet. I'll try to get that in before 1.81.

[urnathan]

Thanks Peter!

[pdimov]

The develop branch has been merged now, so this should be resolved.