https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108089
Bug ID: 108089 Summary: False positive for -Wfree-nonheap-object when using std::variant Product: gcc Version: 12.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: falemagn at gmail dot com Target Milestone: --- The following is the most reduced example I could come up with. #include <variant> #include <string> #include <vector> using variant = std::variant< std::string, std::vector<char> >; extern std::size_t &ext_index; void func() { ext_index = variant(std::string()).index(); } "ext_index" being a reference is all it takes to trigger the issue. If it'd be an object, the issue would not triggered. The issue would be triggered also if ext_index were a pointer. As far as I could tell, ff "std::string" is substituted with a type whose destructor doesn't involve any deallocation, then the issue is not triggered. The issue is also not triggered if std::string is substituted with another std::vector specialization. You can see it all on godbolt: https://godbolt.org/z/aaxY1jW1q This is the error (unecessary lines removed): In member function 'void std::__new_allocator<_Tp>::deallocate(_Tp*, size_type) [with _Tp = char]', inlined from 'static void std::allocator_traits<std::allocator<_CharT> >::deallocate(allocator_type&, pointer, size_type) [with _Tp = char]' at /opt/compiler-explorer/gcc-12.1.0/ [...] inlined from 'std::variant<_Types>::~variant() [with _Types = {std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::vector<char, std::allocator<char> >}]' at /opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/variant:1407:28, inlined from 'void func()' at <source>:14:17: /opt/compiler-explorer/gcc-12.1.0/include/c++/12.1.0/bits/new_allocator.h:158:33: error: 'void operator delete(void*, std::size_t)' called on unallocated object '<anonymous>' [-Werror=free-nonheap-object] 158 | _GLIBCXX_OPERATOR_DELETE(_GLIBCXX_SIZED_DEALLOC(__p, __n)); | ^ <source>: In function 'void func()': <source>:14:38: note: declared here 14 | ext_index = variant(std::string()).index(); | ^