remove_const from value_type of Range::value_type

[tchaikov]

in general, value_type in a range-alike type should not be qualified with const specifier. the reference type does.

before this change, when formatting StringPiece, fmt 11 does not consider it as a formattable type, as it falls into the string-alike category, but its value_type is const char which is different from the Char type of the context, which is char when formatting using fmt::format("{}", ipSlashCidr).

hence we have the build failure like:

/usr/include/fmt/base.h: In instantiation of ‘constexpr fmt::v11::detail::value<Context> fmt::v11::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v11::context; T = folly::Range<const char*>; typename std::enable_if<PACKED, int>::type <anonymous> = 0]’:
/usr/include/fmt/base.h:2002:74:   required from ‘constexpr fmt::v11::detail::format_arg_store<Context, NUM_ARGS, 0, DESC> fmt::v11::make_format_args(T& ...) [with Context = context; T = {folly::Range<const char*>}; long unsigned int NUM_ARGS = 1; long unsigned int NUM_NAMED_ARGS = 0; long long unsigned int DESC = 15; typename std::enable_if<(NUM_NAMED_ARGS == 0), int>::type <anonymous> = 0]’
 2002 |   return {{detail::make_arg<NUM_ARGS <= detail::max_packed_args, Context>(
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
 2003 |       args)...}};
      |       ~~~~~
/usr/include/fmt/format.h:4357:44:   required from ‘std::string fmt::v11::format(format_string<T ...>, T&& ...) [with T = {folly::Range<const char*>&}; std::string = std::__cxx11::basic_string<char>; format_string<T ...> = basic_format_string<char, folly::Range<const char*>&>]’
 4357 |   return vformat(fmt, fmt::make_format_args(args...));
      |                       ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
/home/kefu/folly/folly/IPAddress.cpp:100:47:   required from here
  100 |     throw IPAddressFormatException(fmt::format(
      |                                    ~~~~~~~~~~~^
  101 |         "Invalid ipSlashCidr specified. Expected IP/CIDR format, got '{}'",
      |         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  102 |         ipSlashCidr));
      |         ~~~~~~~~~~~~
/usr/include/fmt/base.h:1611:17: error: static assertion failed: Mixing character types is disallowed.
 1611 |   static_assert(formattable_char, "Mixing character types is disallowed.");
      |                 ^~~~~~~~~~~~~~~~
/usr/include/fmt/base.h:1611:17: note: ‘formattable_char’ evaluates to false
/usr/include/fmt/base.h: In instantiation of ‘constexpr fmt::v11::detail::value<Context> fmt::v11::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v11::context; T = const folly::Range<const char*>; typename std::enable_if<PACKED, int>::type <anonymous> = 0]’:
/usr/include/fmt/base.h:2002:74:   required from ‘constexpr fmt::v11::detail::format_arg_store<Context, NUM_ARGS, 0, DESC> fmt::v11::make_format_args(T& ...) [with Context = context; T = {const folly::Range<const char*>}; long unsigned int NUM_ARGS = 1; long unsigned int NUM_NAMED_ARGS = 0; long long unsigned int DESC = 15; typename std::enable_if<(NUM_NAMED_ARGS == 0), int>::type <anonymous> = 0]’
 2002 |   return {{detail::make_arg<NUM_ARGS <= detail::max_packed_args, Context>(
      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
 2003 |       args)...}};
      |       ~~~~~
/usr/include/fmt/format.h:4357:44:   required from ‘std::string fmt::v11::format(format_string<T ...>, T&& ...) [with T = {const folly::Range<const char*>&}; std::string = std::__cxx11::basic_string<char>; format_string<T ...> = basic_format_string<char, const folly::Range<const char*>&>]’
 4357 |   return vformat(fmt, fmt::make_format_args(args...));
      |                       ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~
/home/kefu/folly/folly/IPAddress.cpp:113:22:   required from here
  113 |           fmt::format("Invalid IP address {}", vec.at(0)));
      |           ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/fmt/base.h:1611:17: error: static assertion failed: Mixing character types is disallowed.
 1611 |   static_assert(formattable_char, "Mixing character types is disallowed.");
      |                 ^~~~~~~~~~~~~~~~
/usr/include/fmt/base.h:1611:17: note: ‘formattable_char’ evaluates to false

in this change, we use the non-const value type, and update the some methods where the reference type is supposed to be used.

Fixes #2172
Signed-off-by: Kefu Chai tchaikov@gmail.com

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[michel-slm]

Looks like we're going with #2265 instead

[tchaikov]

@michel-slm thanks, i still think the std::iterator_traits<Iter>::value_type is a better solution. but please feel free to close this one once #2265 lands.

[vitaut]

Superseded by 21e8dcd but thanks anyway.