Method string_view

test/base-test.cc:897–899

  struct nondeterministic_format_string {
    mutable int i = 0;
    FMT_CONSTEXPR operator fmt::string_view() const {
      return {"{}", i++ != 0 ? 2u : 0u};
    }
  };

#if !FMT_GCC_VERSION || FMT_GCC_VERSION >= 1200