Document std::optional<T> direct-init/copy-init limitation with null

Add regression test pinning current behavior (CHECK_THROWS_AS) in the null
section of unit-conversions.cpp with detailed comment explaining the C++
language-level cause (std::optional's own converting constructor wins
overload resolution over basic_json::operator T()).

Add a warning callout in conversions.md documenting that direct construction/
assignment of std::optional<T> from JSON null throws type_error 302, with a
clear workaround (use get<std::optional<T>>() or get_to() instead, which
correctly produce std::nullopt).

This is a limitation at the language level: there is no SFINAE path to
distinguish "called from inside std::optional's own constructor" from "direct
call", so fixing it would require breaking changes to operator ValueType().
A permanent fix belongs in the 4.0 type-strictness redesign (#3453).

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Code <noreply@anthropic.com>

tests/src/unit-conversions.cpp

@@ -1771,6 +1771,16 @@ TEST_CASE("std::optional")
 
         CHECK(json(opt_null) == j_null);
         CHECK(j_null.get<std::optional<std::string>>() == std::nullopt);
+
+        // Constructing std::optional<T> directly from JSON null throws because
+        // std::optional's own converting constructor is chosen over basic_json's
+        // operator T(). This is a language-level limitation (std::optional<T> is
+        // constructible from T, and T is constructible from basic_json via the
+        // operator); there is no SFINAE path that distinguishes "call from inside
+        // std::optional's constructor" from "direct call". Use get<std::optional<T>>()
+        // or get_to() instead for correct null handling. See json#4864 and #XXXX.
+        CHECK_THROWS_AS(std::optional<std::string>(j_null), json::type_error);
+        CHECK_THROWS_AS(std::optional<int>(j_null), json::type_error);
     }
 
     SECTION("string")