[sayed0200]

Description

Closes #5637.

std::optional<std::reference_wrapper<T>> segfaults when used as a function argument, even though the plain std::reference_wrapper<T> and std::optional<T> forms both work. optional_caster::load() built its inner value caster as a frame-local, emplaced the loaded value into the optional, then let that local go out of scope when the function returned. For a reference_wrapper the loaded value is a reference into the inner caster's own storage, which is exactly where the T lives whenever it has to be materialized from Python (an Eigen::MatrixXd, a std::string, an arithmetic value); once the inner caster was destroyed that reference dangled, so the first read inside the bound call was a heap-use-after-free. I reproduced it under AddressSanitizer and found the same shape in variant_caster, whose per-alternative casters were also frame-locals. The fix keeps the inner caster(s) alive for as long as the composite caster lives, which is the duration of the bound call: optional_caster now holds its inner caster as a member, and variant_caster holds a std::tuple of its alternative casters and indexes into it. The new tests round-trip a long, heap-allocated std::string through std::optional<std::reference_wrapper<std::string>> and std::variant<int, std::reference_wrapper<std::string>>; both fail on master and pass with this change, and the rest of the test_stl suite stays green.

Suggested changelog entry:

• Fixed a dangling reference (use-after-free) when passing std::optional or std::variant of std::reference_wrapper as an argument, where the referenced value is materialized by its caster.