function f1<T>(o: T) {
  o.toString(); // error
  // This is expected, because T might be undefined or null
}

function f2<T extends {}>(o: T) {
  o.toString(); // no error
  // This is expected, this is the correct way to fix the above function.
}

function user<T>(t: T) {
  f1(t);  // Allowed, but irrelevant because f1() is invalid
  f2(t);  // Allowed?  <- this is the bug
  t.toString();  // Error, expected for the same reason as f1()
}