Why doesn't the code compile when connecting vector?

Question

C++ / C#

When including the vector library, the code does not compile with an error in several files, for example, alloc_traits.h on the line namespace __gnu_cxx _GLIBCXX_VISIBILITY(default)
The error itself looks like this: error:namespace definition is not allowed here
What could be the problem here?

Answer 1

Most likely, somewhere in the previous header - in what is in front of ours <vector> - something is not closed (namespace, function, etc.)
UPD2. It turned out to repeat the error with this code.

void x() {
#include <vector>
};

If instead of the x() function we take a class or namespace, the error will be different.
So 90% that you didn't balance the parentheses in the function.