diff --git a/tests/src/unit-allocator.cpp b/tests/src/unit-allocator.cpp
index ca215751e..3c719bf26 100644
--- a/tests/src/unit-allocator.cpp
+++ b/tests/src/unit-allocator.cpp
@@ -14,6 +14,7 @@ using nlohmann::json;
 
 #include <functional>
 #include <limits>
+#include <type_traits>
 #include <valarray>
 #if JSON_HAS_RANGES
     #include <ranges>
@@ -1073,12 +1074,12 @@ struct sweep_allocator
         return (std::numeric_limits<std::size_t>::max)() / sizeof(T);
     }
 
-    // p may point to a const object when libc++ probes whether construct() is
-    // usable for copying, which static_cast<void*>(p) would not accept
-    template<class U, class... Args>
-    void construct(U* p, Args&& ... args) noexcept(noexcept(::new (const_cast<void*>(static_cast<const volatile void*>(p))) U(std::forward<Args>(args)...)))
+    // libc++ probes whether construct() is usable for const objects (when
+    // checking for trivial copies); this must not be viable then
+    template<class U, class... Args, typename = typename std::enable_if<!std::is_const<U>::value>::type>
+    void construct(U* p, Args&& ... args) noexcept(noexcept(::new (static_cast<void*>(p)) U(std::forward<Args>(args)...)))
     {
-        ::new (const_cast<void*>(static_cast<const volatile void*>(p))) U(std::forward<Args>(args)...);
+        ::new (static_cast<void*>(p)) U(std::forward<Args>(args)...);
     }
 
     template<class U>