DOC: notes regarding the current level of C++11 support

doc/technical/code/c++11.txt

@@ -93,3 +93,47 @@ ASCII 'A' = 65 defined: 1 undefd; 0 bool-convertible: 0 can build bool: 1 bool f
 ----
 
 
+
+Known bugs and tricky situations
+--------------------------------
+Summer 2014::
+  the basic switch to C++11 compilation is done by now, but we have yet to deal with
+  some ``ripple effects''.
+September 2014::
+  and those problems turn out to be somewhat insidious: our _reference system_ is still Debian/stable,
+  which means we're using *GCC-4.7* and *CLang 3.0*. While these compilers both provide a roughly complete
+  C++11 support, a lot of fine points were discovered in the follow-up versions of the compilers and standard
+  library -- current versions being GCC-3.9 and CLang 3.4
+  
+  * GCC-4.7 was too liberal and sloppy at some points, where 4.8 rightfully spotted conceptual problems
+  * CLang 3.0 turns out to be really immature and even segfaults on some combinations of new language features
+  * Thus we've got some kind of a _situation:_ We need to hold back further modernisation and just hope that
+    GCC-4.7 doesn't blow up; CLang is even worse, Version 3.0 us unusable after our C++11 transition.
+    We're forced to check our builds on Debian/testing, and we should consider to _raise our general
+    requirement level_ soon.
+
+Perfect forwarding
+~~~~~~~~~~~~~~~~~~
+The ``perfect forwarding'' technique in conjunction with variadic templates promises to obsolete a lot of
+template trickery when it comes to implementing custom containers, allocators or similar kinds of managing wrappers.
+Unfortunately, we ran into nasty problems with both GCC-4.7 and CLang 3.0 here, when chaining several forwarding calls.
+
+- the new _reference collapsing rules_ seem to be unreliably still. Note that even the standard library uses an
+  overload to implement `std::forward`, while in theory, a single definition should work for every case.
+- in one case, the executable generated by GCC passed a reference to an temporary, where it should have
+  passed a rvalue reference (i.e. it should have _moved_ the temporary, instead of referring to the
+  location on stack)
+- CLang is unable to pass a plain-flat rvalue through a chain of templated functions with rvalue references.
+  We get the inspiring error message ``binding of reference to type `std::basic_string<char>` to a value of
+  type `std::basic_string<char>` drops qualifiers''
+
+Thus -- as of 9/2014 -- the _rules of the game_ are as folows
+
+- it is OK to take arguments by rvalue reference, when the type is explicit
+- it is OK to use std::forward _once_ to pass-trough a templated argument
+- but the _time is not yet ready_ to get rid of intermediary copies
+- we still prefer returning by value (eligible for RVO) and copy-initialisation
+- we refrain from switching our metaprogramming code from Loki-Typelists and hand-written specialisations
+  to variadic templates and `std::tuple`
+
+