Compiling code include Eigen library needs permission

I just copy the Eigen folder to /usr/local/include according to this post, to use Eigen library.

Now to compile a simple example it needs to compile with sudo:

simple1.cpp:2:23: fatal error: /usr/local/include/Eigen/Dense: Permission denied

What's wrong? I think it should not ask for permission. Can I fix it?

Edit

I also changed the permission of folder to drwxr-xr-x and added

export LD_LIBRARY_PATH="/usr/local/include/Eigen/"

and log out once. Still, I need permission to compile.

namei -lx /usr/local/include/Eigen/Dense
f: /usr/local/include/Eigen/Dense
Drwxr-xr-x root root /
drwxr-xr-x root root usr
drwxr-xr-x root root local
drwxr-xr-x root root include
drwxr-xr-x root root Eigen
-rw-r--r-- root root Dense

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