https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89461
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This might help, but probably isn't the right fix:
--- a/libstdc++-v3/include/experimental/io_context
+++ b/libstdc++-v3/include/experimental/io_context
@@ -134,7 +134,8 @@ inline namespace v1
io_context* _M_ctx;
};
- using count_type = size_t;
+ using count_type
+ = conditional_t<atomic<size_t>::is_always_lock_free(), size_t, int>;
// construct / copy / destroy:
