Fix py::cast(std::shared_ptr<T>) for shared_ptr-compatible custom holders#6065
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Fix py::cast(std::shared_ptr<T>) for shared_ptr-compatible custom holders#6065virtuald wants to merge 3 commits into
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This restores support for custom holders that are constructible from std::shared_ptr<T> without requiring user code changes or an internals ABI bump. The fix keeps the safety check added in pybind#6008 for incompatible custom holders, but adds an internal path for compatible ones: - py::cast(std::shared_ptr<T>) now recognizes bound types using custom holders - it tunnels shared ownership through an internal thread-local payload - non-smart-holder init_instance() consumes that payload and constructs the bound holder when Holder<T> is constructible from std::shared_ptr<T> This preserves the regression fix for unsafe cases while allowing established patterns such as private-destructor types wrapped by shared_ptr-backed custom holders to keep working.
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May 19, 2026
Lock down the current behavior discussed in issue #6064: py::cast() from std::shared_ptr<T> remains rejected for custom holder bindings that are not std::shared_ptr or py::smart_holder. This extracts the incompatible-holder coverage from PR #6068 and adds a test shaped like issue #6064, while PR #6065 and PR #6066 explore alternative support paths.
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Description
Fixes #6064. I don't really like this approach, it seems like a lot of obscure code to support an edge case, and the thread_local is hard to follow.
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