foo.cc: In function ‘bign& operator++(bign&, int)’:
foo.cc:246:12: warning: reference to local variable ‘t’ returned [-Wreturn-local-addr]
  246 |     return t;
      |            ^
foo.cc:244:10: note: declared here
  244 |     bign t=a;
      |          ^
foo.cc: In function ‘bign& operator--(bign&, int)’:
foo.cc:257:12: warning: reference to local variable ‘t’ returned [-Wreturn-local-addr]
  257 |     return t;
      |            ^
foo.cc:255:10: note: declared here
  255 |     bign t=a;
      |          ^