Fix static_pointer_cast build failure with virtual inheritance in holder_caster_foreign_helpers.h#6014
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oremanj merged 2 commits intopybind:masterfrom Mar 27, 2026
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…irtual inheritance When a class uses virtual inheritance and its holder type is shared_ptr, passing a shared_ptr of the derived type as a method argument triggers a compilation error because static_pointer_cast cannot downcast through a virtual base (dynamic_pointer_cast is needed instead). Made-with: Cursor
…esft downcast
Replace the unconditional static_pointer_cast in set_via_shared_from_this
with a SFINAE-dispatched esft_downcast helper that falls back to
dynamic_pointer_cast when static_cast through a virtual base is ill-formed.
Also add a workaround in the test binding (.def("name") on SftVirtDerived2)
for a separate pre-existing issue with inherited method dispatch through
virtual bases.
Made-with: Cursor
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Summarizing the CI results for the new test, without a fix: https://github.com/pybind/pybind11/actions/runs/23617176855?pr=6014 Every platform that compiles the tests fails with the exact same
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rwgk
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static_pointer_cast build failure with virtual inheritance in holder_caster_foreign_helpers.h
oremanj
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Mar 27, 2026
rwgk
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Mar 30, 2026
…holder_caster_foreign_helpers.h` (pybind#6014) * Add regression test for pybind#5989: static_pointer_cast fails with virtual inheritance When a class uses virtual inheritance and its holder type is shared_ptr, passing a shared_ptr of the derived type as a method argument triggers a compilation error because static_pointer_cast cannot downcast through a virtual base (dynamic_pointer_cast is needed instead). Made-with: Cursor * Fix pybind#5989: use dynamic_pointer_cast for virtual inheritance in esft downcast Replace the unconditional static_pointer_cast in set_via_shared_from_this with a SFINAE-dispatched esft_downcast helper that falls back to dynamic_pointer_cast when static_cast through a virtual base is ill-formed. Also add a workaround in the test binding (.def("name") on SftVirtDerived2) for a separate pre-existing issue with inherited method dispatch through virtual bases. Made-with: Cursor
rwgk
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static_pointer_cast_where_dynamic_pointer_cast_is_needed
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When a class uses virtual inheritance and its holder type is
shared_ptr, any method taking ashared_ptrof the derived type as an argument fails to compile. This is becauseset_via_shared_from_thisinholder_caster_foreign_helpers.hunconditionally usesstatic_pointer_castto downcast from theenable_shared_from_thisbase to the target type. With virtual inheritance,static_castfrom base to derived is ill-formed.The fix introduces a SFINAE-dispatched
esft_downcasthelper with two overloads (using the "tag dispatch via overload priority" trick):int): usesstatic_pointer_cast— selected whenstatic_cast<type*>(esft_base*)is well-formed (non-virtual inheritance). This is the common case, with zero behavioral change from before....): usesdynamic_pointer_cast— selected via SFINAE when thestatic_castis ill-formed (virtual inheritance).Why not simpler alternatives?
dynamic_pointer_castunconditionally: requires polymorphic types (at least one virtual function). Would break existing code that usesenable_shared_from_thiswith non-polymorphic type hierarchies.std::is_polymorphicdispatch: orthogonal to virtual inheritance. A type can be polymorphic without virtual inheritance (common case), so this would unnecessarily usedynamic_pointer_castfor all polymorphic types. There is no standardis_virtual_base_oftrait.Note on the test
The test binds
.def("name", &SftVirtDerived2::name)directly on the derived class rather than relying on inheritance fromSftVirtBase. This works around a separate pre-existing issue where pybind11's method dispatch through virtual inheritance chains uses incorrect pointer offsets, causing a segfault. That issue is unrelated to #5989 and is flagged with aTODOcomment in the test code.CI verification
The first commit (test only, no fix) was pushed to confirm the compilation error reproduces on all CI platforms:
cannot convert from pointer to base class 'SftVirtBase' to pointer to derived class 'SftVirtDerived2' via virtual base 'SftVirtDerived'error C2635: cannot convert a 'SftVirtBase*' to a 'SftVirtDerived2*'; conversion from a virtual base class is impliedEvery platform that compiles tests failed with the same error.
See: https://github.com/pybind/pybind11/actions/runs/23617176855?pr=6014