-Wdangling support for containers with pointer-like elements

[hokein]

It would be great if clang can diagnose the following cases:

void test() {
  std::vector<std::string_view> t1;
  abc.push_back(std::string()); // now t1 holds a dangling pointer. #1


  std::optional<std::string_view> t2 = std::string(); // dangling  #2
  t2 = std::string();  // dangling
}

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

It would be great if clang can diagnose the following cases:

void test() {
  std::vector<std::string_view> t1;
  abc.push_back(std::string()); // now t1 holds a dangling pointer. #<!-- -->1


  std::optional&lt;std::string&gt; t2 = std::string(); // dangling  #<!-- -->2
  t2 = std::string();  // dangling
}

[chrchr-github]

t1 is caught by bugprone-dangling-handle.
https://godbolt.org/z/qa6Werqxo

t2 has a typo, but this is still not caught:

  std::optional<std::string_view> t2 = std::string(); // dangling  #2
  t2 = std::string();  // dangling

[vulptex]

still doesn't catch all cases:

https://godbolt.org/z/f9sWTqjvG

#include <string_view>
#include <string>
#include <vector>
#include <iostream>

void add(std::vector<std::string_view> &vec, int value) {
    auto str = std::to_string(value);
    vec.push_back(str);
}

int main(){

    std::vector<std::string_view> vec;
    add(vec, 1);
    add(vec, 2);
    add(vec, 3);

    for(auto& a: vec){
        std::cout << a << std::endl;
    }

    return 0;
}

[chrchr-github]

The third original example is also not caught:

int test() {
  std::optional<std::string_view> t2;
  t2 = std::string("abc");
  return (*t2).at(0);
}
int main() {
    return test();
}

https://godbolt.org/z/qGbabT6dK