After uninstall and reinstall of xcode (version 2409 reported before & after) I can compile hello-world using clang++ and g++

I am seeing this on my Mac which I've just upgraded to Sequoia: $ cat hello.cpp #include <iostream> int main() { std::cout << "Hello, world!!\n"; } $ clang hello.cpp hello.cpp:1:10: fatal error: 'iostream' file not found 1 | #include <iostream> | ^~~~~~~~~~ 1 error generated. I have one Mac with Sonoma where this works; the above error is from my other Mac where I just installed Sequoia. I do C++ development (on multiple platforms) for a living -- the only change is my having installed Sequoia on my one Mac, whereat a hello-world C++ program no longer compiles.