The sole purpose of this is more type-safe macros.
An alternative solution would be to define a function instead of a
macro. But if the function is unused (currently!) you get a compiler
warning (on some compilers even when marking the function as "static
inline", if it's in the source file). A workaround for that would be
to mark the function as _nm_unused, or to use a macro instead.
_NM_ENSURE_TYPE_CONST() is to aid the macro solution.
(cherry picked from commit 17dc6a9da6)