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Niels Lohmann
2f383b80ee
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>
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