fix: returning a copyable type with a deleted move constructor - #6143
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henryiii merged 2 commits intoAug 14, 2026
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…d#6142) detail::function_ref's constructor SFINAE required is_convertible<Ret, Ret>, which is false for a copyable type whose move constructor is explicitly deleted (the trait tests conversion from an xvalue). In C++17, such a prvalue return is legal via guaranteed copy elision, so also accept an exact type match. Regression introduced in 3.1.0 by the call_impl outlining (pybind#5887). Fixes pybind#6142 Assisted-by: ClaudeCode:claude-fable-5
Skylion007
reviewed
Aug 12, 2026
Per review: the feature-test macro expresses the intent more precisely than PYBIND11_CPP17. Assisted-by: ClaudeCode:claude-opus-5
Skylion007
approved these changes
Aug 14, 2026
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Description
Fixes #6142, a 3.1.0 regression from the
call_imploutlining in #5887.detail::function_ref's constructor SFINAE requiresis_convertible<decltype(f(args...)), Ret>. For a copyable type whose move constructor is explicitly deleted,is_convertible<Ret, Ret>is false, because the trait tests conversion from an xvalue, which selects the deleted move constructor. In C++17 such a prvalue return is legal via guaranteed copy elision, so the constraint rejected code the function body compiles fine. The fix also accepts an exact type match (C++17 and later only; in C++14 such a type cannot be returned by value at all).The regression test fails to compile without the fix and is guarded for C++17, since the C++14 CI builds cannot express the case.
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