[libc++][format] Makes format_context copyable.

[Mordante]

Summary

This was a bug discovered by @jwakely.

[ldionne]

LGTM w/ a proper regression test for this issue.

libcxx/include/__format/format_context.h

88–89

Can we add tests to make sure that it is copyable iff the output iterator is copyable (which means we should also ensure it's not copyable when the output iterator isn't).

[Mordante]

Rebased and addresses review comments.

libcxx/include/__format/format_context.h

class
     // clang-format on
     basic_format_context {
 public:
   using iterator = _OutIt;
   using char_type = _CharT;
   template <class _Tp>
   using formatter_type = formatter<_Tp, _CharT>;
 
-  basic_format_context(const basic_format_context&) = delete;
-  basic_format_context& operator=(const basic_format_context&) = delete;
-
   _LIBCPP_HIDE_FROM_ABI basic_format_arg<basic_format_context>
   arg(size_t __id) const noexcept {
     return __args_.get(__id);
   }
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
   _LIBCPP_HIDE_FROM_ABI _VSTD::locale locale() {
     if (!__loc_)
       __loc_ = _VSTD::locale{};

libcxx/test/std/utilities/format/format.formatter/format.context/format.context/ctor.pass.cpp

 //                      basic_format_args<basic_format_context> args);
 
 #include <format>
 #include <cassert>
 #include <type_traits>
 
 #include "test_basic_format_arg.h"
 #include "test_format_context.h"
+#include "test_iterators.h"
 #include "make_string.h"
 #include "platform_support.h" // locale name macros
 #include "test_macros.h"
 
 template <class OutIt, class CharT>
 void test() {
-  static_assert(
-      !std::is_copy_constructible_v<std::basic_format_context<OutIt, CharT>>);
-  static_assert(
-      !std::is_copy_assignable_v<std::basic_format_context<OutIt, CharT>>);
-  // The move operations are implicitly deleted due to the
-  // deleted copy operations.
-  static_assert(
-      !std::is_move_constructible_v<std::basic_format_context<OutIt, CharT>>);
-  static_assert(
-      !std::is_move_assignable_v<std::basic_format_context<OutIt, CharT>>);
-
   std::basic_string<CharT> string = MAKE_STRING(CharT, "string");
   // The type of the object is an exposition only type. The temporary is needed
   // to extend the lifetype of the object since args stores a pointer to the
   // data in this object.
   auto format_arg_store = std::make_format_args<std::basic_format_context<OutIt, CharT>>(true, CharT('a'), 42, string);
   std::basic_format_args args = format_arg_store;
 
   {
std::locale fr_FR{LOCALE_fr_FR_UTF_8};
     assert(output.front() == CharT('a'));
 
     assert(context.locale() == fr_FR);
     assert(context.locale() != en_US);
   }
 #endif
 }
 
+// std::back_insert_iterator<std::string>, copyable
+static_assert(std::is_copy_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+static_assert(std::is_copy_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+
+static_assert(std::is_move_constructible_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+static_assert(std::is_move_assignable_v<std::basic_format_context<std::back_insert_iterator<std::string>, char>>);
+
+// cpp20_output_iterator, move only
+static_assert(!std::is_copy_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+static_assert(!std::is_copy_assignable_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+
+static_assert(std::is_move_constructible_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+static_assert(std::is_move_assignable_v<std::basic_format_context<cpp20_output_iterator<int*>, char>>);
+
 int main(int, char**) {
   test<std::back_insert_iterator<std::basic_string<char>>, char>();
 #ifndef TEST_HAS_NO_WIDE_CHARACTERS
   test<std::back_insert_iterator<std::basic_string<wchar_t>>, wchar_t>();
 #endif
 
   return 0;
 }