https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366
Bug ID: 100366 Summary: spurious warning - std::vector::clear followed by std::vector::insert(vec.end(), ...) with -O2 Product: gcc Version: 11.1.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: mr...@courier-mta.com Target Milestone: --- I'm seeing this with both gcc 11.1 on: https://godbolt.org/z/1Wv9jj3r1 and gcc (GCC) 11.0.1 20210324 (Red Hat 11.0.1-0), below: Compiling the following with -O2 -Wall -Werror: #include <vector> #include <memory> static char UTC[4]; void func(std::vector<char> &vec) { vec.clear(); vec.insert(vec.end(), UTC, UTC+4); } This generates a massive complaint that boils down to: /usr/include/c++/11/bits/stl_algobase.h:431:30: error: ‘void* __builtin_memcpy(v oid*, const void*, long unsigned int)’ writing 1 or more bytes into a region of size 0 overflows the destination [-Werror=stringop-overflow=] 431 | __builtin_memmove(__result, __first, sizeof(_Tp) * _Num); | ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In file included from /usr/include/c++/11/x86_64-redhat-linux/bits/c++allocator. ...skipping 1 line from /usr/include/c++/11/bits/allocator.h:46, from /usr/include/c++/11/vector:64, from t.C:1: Removing vec.clear() makes this go away. Adding vec.reserve(4) after vec.clear() makes this go away.