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

This was a bug discovered by @jwakely.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D137911

Author: mordante

libcxx/include/__format/format_context.h

@@ -85,9 +85,6 @@ class
   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);

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

@@ -28,23 +28,13 @@
 
 #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
@@ -118,6 +108,20 @@ void test() {
 #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