Fix issue with constructing an FOA container from a class with a template conversion operator (#334)

* Add missing compile-only tests in CMake

* Rewrite the FOA containers' CFOA constructors so that a class with a template conversion operator does not instantiate the CFOA container

* Rewrite the CFOA containers' FOA constructors so that a class with a template conversion operator does not instantiate the FOA container

include/boost/unordered/concurrent_flat_set.hpp

@@ -2,6 +2,7 @@
  *
  * Copyright 2023 Christian Mazakas.
  * Copyright 2023-2026 Joaquin M Lopez Munoz.
+ * Copyright 2026 Braden Ganetsky
  * Distributed under the Boost Software License, Version 1.0.
  * (See accompanying file LICENSE_1_0.txt or copy at
  * http://www.boost.org/LICENSE_1_0.txt)
@@ -186,10 +187,10 @@ namespace boost {
       {
       }
 
-
-      template <bool avoid_explicit_instantiation = true>
+      template <typename Key2,
+        typename std::enable_if<std::is_same<Key, Key2>::value, int>::type = 0>
       concurrent_flat_set(
-        unordered_flat_set<Key, Hash, Pred, Allocator>&& other)
+        unordered_flat_set<Key2, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))
       {
       }