https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63811
--- Comment #1 from howarth at bromo dot med.uc.edu ---
The change...
Index: libstdc++-v3/include/bits/stl_deque.h
===================================================================
--- libstdc++-v3/include/bits/stl_deque.h (revision 217331)
+++ libstdc++-v3/include/bits/stl_deque.h (working copy)
@@ -642,7 +642,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
// Create a copy of the current allocator.
_Tp_alloc_type __alloc{_M_get_Tp_allocator()};
// Put that copy in a moved-from state.
- _Tp_alloc_type __unused __attribute((__unused__)) {std::move(__alloc)};
+ _Tp_alloc_type __unused__ __attribute((__unused__)) {std::move(__alloc)};
// Create an empty map that allocates using the moved-from allocator.
_Deque_base __empty{__alloc};
// Now safe to modify current allocator and perform non-throwing swaps.
eliminates the bootstrap error
