Fix UB in format_arg_store implementation.

format_arg_store struct contains pointers to its own members, so autogenerated
copy constructor and copy assignment is not correct. Need to reassign pointers
to |this| internals.

include/fmt/base.h

@@ -1636,7 +1636,9 @@ template <typename Context, size_t NUM_ARGS, size_t NUM_NAMED_ARGS,
 struct format_arg_store {
   // args_[0].named_args points to named_args to avoid bloating format_args.
   // +1 to workaround a bug in gcc 7.5 that causes duplicated-branches warning.
-  arg_t<Context, NUM_ARGS> args[1 + (NUM_ARGS != 0 ? NUM_ARGS : +1)];
+  static constexpr int ARGS_ARR_SIZE = 1 + (NUM_ARGS != 0 ? NUM_ARGS : +1);
+
+  arg_t<Context, NUM_ARGS> args[ARGS_ARR_SIZE];
   named_arg_info<typename Context::char_type> named_args[NUM_NAMED_ARGS];
 
   template <typename... T>
@@ -1649,6 +1651,25 @@ struct format_arg_store {
         0,
         (init_named_arg(named_args, arg_index, named_arg_index, values), 0)...};
   }
+
+  format_arg_store(const format_arg_store& rhs) {
+    args[0] = {named_args, NUM_NAMED_ARGS};
+    for (int i = 1; i < ARGS_ARR_SIZE; ++i)
+      args[i] = rhs.args[i];
+    for (int i = 0; i < NUM_NAMED_ARGS; ++i)
+      named_args[i] = rhs.named_args[i];
+  }
+
+  format_arg_store &operator=(const format_arg_store &rhs) {
+    if (this != &rhs) {
+      args[0] = {named_args, NUM_NAMED_ARGS};
+      for (int i = 1; i < ARGS_ARR_SIZE; ++i)
+        args[i] = rhs.args[i];
+      for (int i = 0; i < NUM_NAMED_ARGS; ++i)
+        named_args[i] = rhs.named_args[i];
+    }
+    return *this;
+  }
 };
 
 // A specialization of format_arg_store without named arguments.