Fix compilation of Boost.Variants move assignment for situations when one of the variant template classes has nothrow copy constructor and throwing move constructor (refs #8772)

Fixes compilation error:

.../output/host/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/variant/variant.hpp: In member function 'void boost::variant<T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19>::move_assigner::internal_visit(RhsT&, int) [with RhsT = boost::shared_ptr<void>, T0_ = boost::shared_ptr<void>, T1 = boost::signals2::detail::foreign_void_shared_ptr, T2 = boost::detail::variant::void_, ..., T18 = boost::detail::variant::void_, T19 = boost::detail::variant::void_]':
...
.../output/host/usr/arm-buildroot-linux-gnueabihf/sysroot/usr/include/boost/variant/variant.hpp:2058:13: error: no matching function for call to 'boost::variant<boost::shared_ptr<void>, boost::signals2::detail::foreign_void_shared_ptr>::move_assigner::assign_impl(boost::shared_ptr<void>&, nothrow_copy, nothrow_move_constructor, boost::variant<boost::shared_ptr<void>, boost::signals2::detail::foreign_void_shared_ptr>::has_fallback_type_)'

Reported here: https://svn.boost.org/trac/boost/ticket/8772

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Backported-from: https://svn.boost.org/trac/boost/changeset/85080

--- a/boost/variant/variant.hpp	(revision 85079)
+++ b/boost/variant/variant.hpp	(revision 85080)
@@ -1981,5 +1981,5 @@
     private: // helpers, for internal visitor interface (below)
 
-        template <typename RhsT, typename B1, typename B2>
+        template <typename RhsT, typename B2>
         void assign_impl(
               RhsT& rhs_content
--- a/libs/variant/test/rvalue_test.cpp	(revision 85079)
+++ b/libs/variant/test/rvalue_test.cpp	(revision 85080)
@@ -197,4 +197,19 @@
 #endif
 
+struct nothrow_copyable_throw_movable {
+    nothrow_copyable_throw_movable(){}
+    nothrow_copyable_throw_movable(const nothrow_copyable_throw_movable&) BOOST_NOEXCEPT {}
+#ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
+    nothrow_copyable_throw_movable(nothrow_copyable_throw_movable&&) BOOST_NOEXCEPT_IF(false) {}
+#endif
+};
+
+// This test is created to cover the following situation:
+// https://svn.boost.org/trac/boost/ticket/8772
+void run_tricky_compilation_test()
+{
+    boost::variant<int, nothrow_copyable_throw_movable> v;
+    v = nothrow_copyable_throw_movable();
+}
 
 int test_main(int , char* [])
@@ -204,4 +219,5 @@
    run_move_only();
    run_moves_are_noexcept();
+   run_tricky_compilation_test();
    return 0;
 }