fix: explicitly check operator[] in is_contiguous

[tearfur]

Prevent false positives when the container itself does not have operator[](size_t), but the target type of one of the non-explicit user-defined conversion functions does has a built-in subscript operator. Edit: corrected wording

Example: https://godbolt.org/z/53bqcn97W

Error message

In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:41,
                 from <source>:3:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h: In instantiation of 'static constexpr void fmt::v12::detail::container_buffer<Container>::grow(fmt::v12::detail::buffer<typename Container::value_type>&, size_t) [with Container = MyBuf<char, 256>; typename Container::value_type = char; size_t = long unsigned int]':
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2029:56:   required from 'fmt::v12::detail::container_buffer<Container>::container_buffer(Container&) [with Container = MyBuf<char, 256>]'
 2029 |       : buffer<value_type>(grow, c.size()), container(c) {}
      |                                                        ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2046:32:   required from 'fmt::v12::detail::iterator_buffer<OutputIt, typename std::enable_if<(fmt::v12::detail::is_back_insert_iterator<OutputIt>::value && fmt::v12::detail::is_contiguous<typename OutputIt::container_type>::value), typename OutputIt::container_type::value_type>::type>::iterator_buffer(OutputIt, size_t) [with OutputIt = std::back_insert_iterator<MyBuf<char, 256> >; typename std::enable_if<(fmt::v12::detail::is_back_insert_iterator<OutputIt>::value && fmt::v12::detail::is_contiguous<typename OutputIt::container_type>::value), typename OutputIt::container_type::value_type>::type = char; typename OutputIt::container_type = MyBuf<char, 256>; typename OutputIt::container_type::value_type = char; class OutputIt::container_type = MyBuf<char, 256>; size_t = long unsigned int]'
 2046 |       : base(get_container(out)) {}
      |                                ^
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2089:10:   required from 'fmt::v12::detail::iterator_buffer<OutputIt, T> fmt::v12::detail::get_buffer(OutputIt) [with T = char; OutputIt = std::back_insert_iterator<MyBuf<char, 256> >; typename std::enable_if<(! is_buffer_appender<OutputIt>::value), int>::type <anonymous> = 0]'
 2089 |   return iterator_buffer<OutputIt, T>(out);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2797:40:   required from 'fmt::v12::remove_cvref_t<T> fmt::v12::vformat_to(OutputIt&&, string_view, format_args) [with OutputIt = std::back_insert_iterator<MyBuf<char, 256> >&; typename std::enable_if<detail::is_output_iterator<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, char>::value, int>::type <anonymous> = 0; remove_cvref_t<T> = std::back_insert_iterator<MyBuf<char, 256> >; string_view = basic_string_view<char>; format_args = basic_format_args<context>]'
 2797 |   auto&& buf = detail::get_buffer<char>(out);
      |                ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2817:20:   required from 'fmt::v12::remove_cvref_t<T> fmt::v12::format_to(OutputIt&&, format_string<T ...>, T&& ...) [with OutputIt = std::back_insert_iterator<MyBuf<char, 256> >; T = {const char (&)[5]}; typename std::enable_if<detail::is_output_iterator<typename std::remove_cv<typename std::remove_reference<_Tp>::type>::type, char>::value, int>::type <anonymous> = 0; remove_cvref_t<T> = std::back_insert_iterator<MyBuf<char, 256> >; format_string<T ...> = fstring<const char (&)[5]>]'
 2817 |   return vformat_to(out, fmt.str, vargs<T...>{{args...}});
      |          ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:50:19:   required from here
   50 |     fmt::format_to(std::back_inserter(buf), "{}", "test");
      |     ~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:2022:14: error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
 2022 |     self.set(&self.container[0], capacity);
      |              ^~~~~~~~~~~~~~~~
      |              |
      |              const char*
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:1770:29: note: initializing argument 1 of 'constexpr void fmt::v12::detail::buffer<T>::set(T*, size_t) [with T = char; size_t = long unsigned int]'
 1770 |   FMT_CONSTEXPR void set(T* buf_data, size_t buf_capacity) noexcept {
      |                          ~~~^~~~~~~~
Compiler returned: 1

[vitaut]

LGTM but please add a test case to base-test.

[vitaut]

Mostly looks good, just two minor comments.

[vitaut]

FMT_NODISCARD is not needed, this is just a test. Also please apply clang-format.

[tearfur]

f07287f I didn't notice the incorrect format because the CI passed. Weird.

[vitaut]

buffer_ -> buffer since it is a public member.

[tearfur]

665587b

[vitaut]

LGTM

[vitaut]

Merged, thanks