clangd doesn't recognize standard headers – Ubuntu

This question has been asked and answered a bunch of times, but none of the answers seem to work for me.

I’ve been trying to get clangd set up in nvim lsp. I used bear to generate compile_commands.json, but clangd still gives me errors telling me it can’t find standard library headers.
Here’s a minimal example:

main.cpp:

#include <iostream>
using namespace std;

int main(){
  cout << "hello clangd";
  return 0;
}

I then run: bear -- g++ main.cpp, which compiles and creates a compile_commands.json with this content:

[
  {
    "arguments": [
      "/usr/bin/g++",
      "-c",
      "main.cpp"
    ],
    "directory": "/home/xxx/tmp/hello_clangd",
    "file": "/home/xxx/tmp/hello_clangd/main.cpp"
  }
]

I also tried compiling using a cmake flag to generate compile_commands.json but I’m getting the same issue. I can get the file but the language server still won’t work properly.

I have been able to use clang with vim-pio so it seems it’s not broken. what am I missing.

EDIT: I’m on ubuntu btw