Method operator=

test/format-test.cc:1176–1179

  // Iterator used to abort the actual output.
  struct throwing_iterator {
    auto operator=(char) -> throwing_iterator& {
      throw std::runtime_error("aborted");
      return *this;
    }
    auto operator*() -> throwing_iterator& { return *this; }
    auto operator++() -> throwing_iterator& { return *this; }
    auto operator++(int) -> throwing_iterator { return *this; }