Accept lvalues in ordered_map::emplace's value parameter

ordered_map::emplace(key, value) took the mapped value only by T&&, an
rvalue reference rather than a forwarding reference, so
ordered_json::emplace("a", value) failed to compile whenever value was
an lvalue or a const lvalue, even though the same call compiles for
json (whose object_t is std::map, with a variadic emplace). Turn the
value parameter into a separately-deduced forwarding reference,
constrained with std::is_constructible so the overloads still only
accept something convertible to the mapped type. std::map-compatible
semantics are unchanged: emplace still does nothing if the key already
exists.

Open PR #5609 also touches ordered_map.hpp (moving values on vector
growth); this change only touches the two emplace() overloads and
should not conflict.

Fixes #5673.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-ordered_json.cpp

@@ -196,3 +196,44 @@ TEST_CASE("regression test - diff() must account for ordered_json member order")
         CHECK(a.patch(p) == b);
     }
 }
+
+TEST_CASE("regression test for issue #5673 - ordered_json::emplace with a non-rvalue value")
+{
+    SECTION("lvalue value")
+    {
+        ordered_json oj = ordered_json::object();
+        ordered_json value = 1;
+        auto res = oj.emplace("a", value);
+        CHECK(res.second == true);
+        CHECK(oj.dump() == "{\"a\":1}");
+    }
+
+    SECTION("const lvalue value")
+    {
+        ordered_json oj = ordered_json::object();
+        const ordered_json value = 1;
+        auto res = oj.emplace("a", value);
+        CHECK(res.second == true);
+        CHECK(oj.dump() == "{\"a\":1}");
+    }
+
+    SECTION("rvalue value")
+    {
+        ordered_json oj = ordered_json::object();
+        auto res = oj.emplace("a", ordered_json(1));
+        CHECK(res.second == true);
+        CHECK(oj.dump() == "{\"a\":1}");
+    }
+
+    SECTION("existing key is not overwritten (std::map-compatible semantics)")
+    {
+        ordered_json oj = ordered_json::object();
+        ordered_json value = 1;
+        oj.emplace("a", value);
+
+        ordered_json other_value = 2;
+        auto res = oj.emplace("a", other_value);
+        CHECK(res.second == false);
+        CHECK(oj.dump() == "{\"a\":1}");
+    }
+}