repro.cpp

// g++ -O2 -std=c++20 -o foo repro.cpp && ./foo
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Vector {
    ~Vector() {}
    char* data() {
        if (m_outline)
            return m_outline;
        return reinterpret_cast<char*>(m_inline);
    }
    char m_inline[32];
    char* m_outline { nullptr };
};
static Vector make_vector() {
    auto buffer = Vector();
    unsigned new_size = 128;
    auto* new_buffer = new char[new_size];
    buffer.m_outline = new_buffer;
    if (!new_buffer) return buffer; // !!!
    for (unsigned i = 0; i < 128; ++i)
        buffer.m_outline[i] = 0;
    // This fixes the warning:
    // if (!buffer.m_outline) __builtin_unreachable();

    // The compiler thinks this will overflow. Why?!
    memset(buffer.data(), 'A', new_size);
    //> warning: ‘void* memset(void*, int, size_t)’ writing 128 bytes into a region of size 40 overflows the destination [-Wstringop-overflow=]

    // Uncomment the following println() and it magically works, *and* shows that it uses the correct buffer. Huh?!
    // printf("vector struct is initially at %p, first datum is at %p\n", &buffer, buffer.data());

    return buffer;
}
int main() {
    auto actual = make_vector();
    printf("vector struct is later at %p, first datum is at %p\n", &actual, actual.data());
    return 0;
}