Don't explicitly delete copy ctor of dynamic_format_arg_store (#2664)

* Don't explicitly delete copy ctor of dynamic_format_arg_store

Explicitly deleting the copy ctor causes the move constructor to not be
implicitly generated. This behaviour is different than what was in
v8.0.1 and causes code that relied on the move ctor of
dynamic_format_arg_store to break.

* Add test for dynamic_format_arg_store's move ctor

* include <memory>, don't use make_unique

include/fmt/args.h

@@ -145,9 +145,6 @@ class dynamic_format_arg_store
  public:
   constexpr dynamic_format_arg_store() = default;
 
-  constexpr dynamic_format_arg_store(
-      const dynamic_format_arg_store<Context>& store) = delete;
-
   /**
     \rst
     Adds an argument into the dynamic store for later passing to a formatting

test/args-test.cc

@@ -7,6 +7,8 @@
 
 #include "fmt/args.h"
 
+#include <memory>
+
 #include "gtest/gtest.h"
 
 TEST(args_test, basic) {
@@ -171,3 +173,21 @@ TEST(args_test, throw_on_copy) {
   }
   EXPECT_EQ(fmt::vformat("{}", store), "foo");
 }
+
+TEST(args_test, move_constructor) {
+  const int test_integer = 42;
+  const char* const test_c_string = "foo";
+
+  std::unique_ptr<fmt::dynamic_format_arg_store<fmt::format_context>>
+      store_uptr(new fmt::dynamic_format_arg_store<fmt::format_context>());
+  store_uptr->push_back(test_integer);
+  store_uptr->push_back(std::string(test_c_string));
+  store_uptr->push_back(fmt::arg("a1", test_c_string));
+
+  fmt::dynamic_format_arg_store<fmt::format_context> moved_store(
+      std::move(*store_uptr));
+  store_uptr.reset();
+  EXPECT_EQ(
+      fmt::vformat("{} {} {a1}", moved_store),
+      std::to_string(test_integer) + " " + test_c_string + " " + test_c_string);
+}