The problem

I’m trying to understand how C++ compilers handle undefined variables, and I’ve written the following code :

int test() {
  bool b; //undefined

  if(b){return 1;}
  else {return 2;}
  return 0;
}

Clang

When I compile with x86_64 clang 18.1.0 with flags -O3 -std=c++23 I get the following assembly

test():                               # @test()

Which means : this function does nothing (it evens doesn’t return).

Gcc

When the same code is compiled with x86_64 gcc 14.1 with flags -O3 -std=c++23 I get the following assemblby :

test():
        mov     eax, 2
        ret

which means : return 2.

My question

I’m confused as I thought that when a variable is undefined, the compiler is allowed to choose a path (based on whatever rule it wants) so I expected either 1 or 2 in the return value as gcc does, but I didn’t explected an invalid function as clang does.

Can you please explain me what’s the standard rule the compiler should follow when they test an undefined variable. And who’s wrong (my expectations, clang, gcc) ?.

I tried to get illumination by drinking a coffee. I’m now caffeinated, not illuminated.