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Description
#include <memory>
class Foo {
inline static std::unique_ptr<Foo> instance_;
};See: https://godbolt.org/z/MqMxs9nzr
This fails to compile due to unique_ptr<T>::~unique_ptr() (and thus default_delete<T>::operator()) being instantiated at the point of the variable declaration (when the class isn't fully defined yet). Since c++23, unique_ptr and default_delete::operator() are constexpr, but the declaration of the unique_ptr should not attempt to fully instantiate the destructor or std::default_delete::operator() yet. Or at least the above usage pattern should be allowed, as this is a valid pattern for singletons (that aren't Meyers style). Should this fall back to non-constexpr unique_ptr/default_delete?
This fails using both libc++ and libstdc++. gcc and msvc both compile it fine.
In file included from test.cpp:1:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/memory:944:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/__memory/inout_ptr.h:16:
In file included from /builds/clang-19.1.0/bin/../include/c++/v1/__memory/shared_ptr.h:32:
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:78:19: error: invalid application of 'sizeof' to an incomplete type 'Foo'
78 | static_assert(sizeof(_Tp) >= 0, "cannot delete an incomplete type");
| ^~~~~~~~~~~
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:292:7: note: in instantiation of member function 'std::default_delete<Foo>::operator()' requested here
292 | __ptr_.second()(__tmp);
| ^
/builds/clang-19.1.0/bin/../include/c++/v1/__memory/unique_ptr.h:261:71: note: in instantiation of member function 'std::unique_ptr<Foo>::reset' requested here
261 | _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX23 ~unique_ptr() { reset(); }
| ^
simple.cpp:4:38: note: in instantiation of member function 'std::unique_ptr<Foo>::~unique_ptr' requested here
4 | inline static std::unique_ptr<Foo> instance;
| ^
simple.cpp:3:7: note: definition of 'Foo' is not complete until the closing '}'
3 | class Foo {