Detail: complex is a struct in Dinkumware.

IIRC it's `class` in SGI's STL and most other implementations. The state
for similar warnings for the compilers that I've looked at:

- Visual C++ never warns because I used a pragma to disable the warning.
- GCC never warns with its standard library since it contains a
  `system_header` pragma. It might warn for other libraries.
- Clang respects GCC's pragma so it never warns for GCC's standard
  library. Forward declarations are disabled for libc++ so this isn't an
  issue there. It does warn when using Dinkumware and possibly other
  standard libraries.

I'm tempted to make container forwarding 'opt-in' rather than 'opt-out'.
Or maybe only enable for compiler/library combinations where it's known
to work.

[SVN r75561]

include/boost/detail/container_fwd.hpp

@@ -63,8 +63,7 @@
 #  elif (defined(_YVALS) && !defined(__IBMCPP__)) || defined(_CPPLIB_VER)
      // Dinkumware Library (this has to appear after any possible replacement
      // libraries)
-     //
-     // Works fine.
+#    define BOOST_CONTAINER_FWD_COMPLEX_STRUCT
 #  else
 #    define BOOST_DETAIL_NO_CONTAINER_FWD
 #  endif
@@ -117,7 +116,11 @@ namespace std
     template <class charT> struct char_traits;
 #endif
 
+#if defined(BOOST_CONTAINER_FWD_COMPLEX_STRUCT)
+    template <class T> struct complex;
+#else
     template <class T> class complex;
+#endif
 
 #if !defined(BOOST_CONTAINER_FWD_BAD_DEQUE)
     template <class T, class Allocator> class deque;

test/container_fwd/Jamfile

@@ -11,6 +11,7 @@ project detail/test/container_fwd
         <toolset>intel:<warnings>on
         <toolset>gcc:<cxxflags>"-pedantic -Wstrict-aliasing -fstrict-aliasing -Wextra -Wsign-promo -Wunused-parameter -Wconversion"
         <toolset>darwin:<cxxflags>"-pedantic -Wstrict-aliasing -fstrict-aliasing -Wextra -Wsign-promo -Wunused-parameter -Wconversion"
+        <toolset>clang:<cxxflags>"-pedantic -Wextra -Wmismatched-tags"
         <warnings-as-errors>on
   ;