fix: returning a copyable type with a deleted move constructor - #6143
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henryiii wants to merge 1 commit into
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fix: returning a copyable type with a deleted move constructor#6143henryiii wants to merge 1 commit into
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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
| // copy elision. Accept that case explicitly. See issue #6142. | ||
| template <typename From, typename To> | ||
| using is_returnable_as = bool_constant< | ||
| #if defined(PYBIND11_CPP17) |
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Guard with feature flag: "__cpp_guaranteed_copy_elision" :) seems more specific here and better expresses intent.
#include <iostream>
int main() {
#if defined(__cpp_guaranteed_copy_elision) && __cpp_guaranteed_copy_elision >= 201606L
std::cout << "Guaranteed prvalue elision is supported!\n";
#else
std::cout << "Old value categories apply (copies/moves may occur).\n";
#endif
}| // #6142: returning a copy-constructible type with an explicitly deleted move | ||
| // constructor failed to compile inside detail::function_ref (3.1.0 regression). | ||
| // Returning such a type by value requires C++17 guaranteed copy elision. | ||
| #if defined(PYBIND11_CPP17) |
| .def_static("get_one", &lacking_move_ctor::get_one, py::return_value_policy::move); | ||
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| // test_copy_only_deleted_move (#6142) | ||
| #if defined(PYBIND11_CPP17) |
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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.
Suggested changelog entry: