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

test/unordered/conversion_operator_tests.cpp

@@ -0,0 +1,46 @@
+// Copyright 2026 Braden Ganetsky
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#if !defined(BOOST_UNORDERED_FOA_TESTS)
+#error "This test is only for the FOA-style conatiners"
+#endif
+
+#include <boost/static_assert.hpp>
+#include <boost/unordered/unordered_flat_map.hpp>
+#include <boost/unordered/unordered_flat_set.hpp>
+#include <boost/unordered/unordered_node_map.hpp>
+#include <boost/unordered/unordered_node_set.hpp>
+// Don't include the CFOA headers here!
+
+using flat_map = boost::unordered::unordered_flat_map<int, int>;
+using flat_set = boost::unordered::unordered_flat_set<int>;
+using node_map = boost::unordered::unordered_node_map<int, int>;
+using node_set = boost::unordered::unordered_node_set<int>;
+
+struct constrained_template_converter
+{
+  struct dummy
+  {
+  };
+  template <class T, typename std::enable_if<
+                       std::is_constructible<T, dummy>::value, int>::type = 0>
+  operator T() const
+  {
+    return T{};
+  }
+};
+
+// Check whether the corresponding CFOA container gets instantiated.
+// The CFOA headers aren't included, so this would fail to compile if the CFOA
+// container was instantiated.
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<flat_map, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<flat_set, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<node_map, constrained_template_converter>::value));
+BOOST_STATIC_ASSERT(
+  (!std::is_constructible<node_set, constrained_template_converter>::value));
+
+int main() { return 0; }