std::vector::operator[] allows out-of-bounds assignment on reserved elements in Clang/libc++ while at() correctly throws

[hutuhutong]

I am not sure whether this behavior needs adjustment. Assigning to a std::vector element via operator[] at an index beyond the current size but within the reserved capacity does not trigger a runtime error in Clang/libc++. In contrast, using v.at() for the same index correctly throws a std::out_of_range exception.

clang20.1.0 -O0 -std=c++20 -stdlib=libc++
========= code ========

#include <vector>
#include <iostream>

int main() {
    std::vector<int> v;
    v.reserve(10); 
    // v.at(3) = 42;    // will trigger the error
    v[3] = 0;   // not trigger the error
    std::cout << v[3] << "\n";
}

========= output ========
when v.at(3) = 42 is executed, the program output;

ASM generation compiler returned: 0
Execution build compiler returned: 0
Program returned: 139
libc++abi: terminating due to uncaught exception of type std::out_of_range: vector
Program terminated with signal: SIGSEGV

when v[3] = 0 is executed, the program output;

ASM generation compiler returned: 0
Execution build compiler returned: 0
Program returned: 0
0

[frederick-vs-ja]

We should diagnose this in some hardening modes. See also #127885.

[philnik777]

We do diagnose this already with any hardening enabled: https://godbolt.org/z/5MbvYdx3c. The experimental library and assertion semantic are only required to get the pretty diagnostic. You can also get that with just -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG without experimental/assertion semantic.