Why are some files in libC ++ include_next themselves?

Question

I looked at the source code for libC ++, and I found a design template that I don't understand: many of the headers include_nextthemselves.

For example, it wchar.hhas line 17:

#include_next <wchar.h>

My understanding include_nextis that it excludes the directory of the current file, so it should include another wchar.h,

Where is this other file located wchar.h?

What search path should I add to build libC ++?

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This is not a duplicate. Why use #include_next in a project? because I am asking about libC ++ specifically.