Workaround visibility bug in gcc-6 by un-nesting lambdas

include/boost/unordered/detail/foa.hpp

@@ -975,6 +975,40 @@ public:
     static constexpr auto pocma=
       alloc_traits::propagate_on_container_move_assignment::value;
 
+    /*  Avoid using nested lambdas with a `this` capture as it seems to trigger
+      * a bug in GCC:
+      * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80947
+      * 
+      * Rather than directly attempting to manipulate the visibility of the
+      * table class, it's easier to work around the bug by simply un-nesting the
+      * lambdas
+      */
+    auto const move_element=[this](value_type* p){
+      unchecked_insert(std::move(*p));
+    };
+
+    auto const elementwise_move=[&,this]{
+      /* The check above is redundant: we're setting up a compile-time
+        * barrier so that the compiler is convinced we're not throwing
+        * under noexcept(true) conditions.
+        */
+
+      reserve(x.size());
+      BOOST_TRY{
+        /* This works because subsequent x.clear() does not depend on the
+          * elements' values.
+          */
+
+        x.for_all_elements(move_element);
+      }
+      BOOST_CATCH(...){
+        x.clear();
+        BOOST_RETHROW
+      }
+      BOOST_CATCH_END
+      x.clear();
+    };
+
     if(this!=&x){
       clear();
       h()=std::move(x.h());
@@ -987,29 +1021,8 @@ public:
         swap(arrays,x.arrays);
         swap(ml,x.ml);
       }
-      else if_constexpr<!alloc_traits::is_always_equal::value>([&,this]{
+      else if_constexpr<!alloc_traits::is_always_equal::value>(
-        /* The check above is redundant: we're setting up a compile-time
+        elementwise_move);
-         * barrier so that the compiler is convinced we're not throwing
-         * under noexcept(true) conditions.
-         */
-
-        reserve(x.size());
-        BOOST_TRY{
-          /* This works because subsequent x.clear() does not depend on the
-           * elements' values.
-           */
-
-          x.for_all_elements([this](value_type* p){
-            unchecked_insert(type_policy::move(*p));
-          });
-        }
-        BOOST_CATCH(...){
-          x.clear();
-          BOOST_RETHROW
-        }
-        BOOST_CATCH_END
-        x.clear();
-      });
     }
     return *this;
   }