rosidl_typesupport_introspection_cpp potentially unsafe

[Karsten1987]

Bug report

Required Info:

• Operating System:
  • All
• Installation type:
  • source
• Version or commit hash:
  • master
• DDS implementation:
  • N/A
• Client library (if applicable):
  • rosidl_typesupport_introspection_cpp

Steps to reproduce issue

The code in question is here:

rosidl/rosidl_typesupport_introspection_cpp/resource/msg__type_support.cpp.em

Lines 70 to 71 in 075fe35

	@(field.type.pkg_name)::msg::@(field.type.type) * member =
	reinterpret_cast<@(field.type.pkg_name)::msg::@(field.type.type) *>(untyped_member);

Fixed size arrays are getting resolved to std::array<T, N>, however the getter function inside the introspection code access the elements via a reinterpret_cast<T *>, which is potentially unsafe to do.
More info here:
https://stackoverflow.com/questions/39376813/is-the-stdarray-bit-compatible-with-the-old-c-array/39379359#39379359

This came out during dev time of rosbag2:
ros2/rosbag2#56 (comment)

Expected behavior

Does it make sense to cast it to an std::array<T, N> before accessing the element?

Actual behavior

Cast to T *