Let’s say I have main.cpp that includes utils.h from lib directory:

lib/utils.h:

#ifndef LIB_UTILS_H
#define LIB_UTILS_H

inline void util() {
  throw 13;
}

#endif // LIB_UTILS_H

main.cpp:

#include "utils.h"

int main() {
  util();
  return 0;
}

If I tell clang to search in lib using -I it respects -fno-exceptions:

% clang++ -I lib -fno-exceptions main.cpp
In file included from main.cpp:1:
lib/utils.h:5:3: error: cannot use 'throw' with exceptions disabled
  throw 13;
  ^
1 error generated.
%

But if I use -isystem instead of -I it compiles successfully:

% clang++ -isystem lib -fno-exceptions main.cpp
%

And when I try running it, it throws and exception:

% ./a.out
terminate called after throwing an instance of 'int'
zsh: IOT instruction  ./a.out

I checked with Clang 14 and 18, they behave the same. gcc correctly fails to compile in both cases.

Why is -fno-exceptions ignored by Clang?