nlohmann / json / 29035700680

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Document std::optional<T> direct construction limitation (#5247)

* 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>

* Fix issue reference in std::optional test comment

Update the comment in the null section test to reference #5246 instead of
placeholder #XXXX, clarifying where the direct-init/copy-init limitation is tracked.

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

* Use CHECK_THROWS_AS_WITH for std::optional test assertions

Update the regression tests to use CHECK_THROWS_AS_WITH instead of
CHECK_THROWS_AS to verify both the exception type and the error message.

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

* Fix CI: use CHECK_THROWS_WITH_AS, the macro that actually exists

CHECK_THROWS_AS_WITH is not a doctest macro; the correct one used throughout
this test suite is CHECK_THROWS_WITH_AS(expr, message, exception_type&), with
the message before the type and the type as a reference. The previous commit
didn't catch this because it only compiled the file standalone with default
settings; this TEST_CASE only... (continued)

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