foo.cc: In function ‘int gcd(int, int)’:
foo.cc:10:5: error: named return values are no longer supported
   10 |     return m==0?n:gcd(m,n%m);
      |     ^~~~~~
foo.cc:23:1: warning: no return statement in function returning non-void [-Wreturn-type]
   23 | }
      | ^