Prevent hijacking a namespace to an inline namespace

Must the declaration of a namespace as inline be the first one, or can any (reopened) namespace be declard as inline at any point?

If so, is there a way to prevent someone to declare a namespace as inline (afterwards)?

#include <iostream>

namespace outer {
//inline
namespace inner {
}}

//inline namespace std {}

namespace outer {
inline
namespace inner {
    void foo() {
        std::cout << "foo" << std::endl;
        //::cout << "foo" << endl;
    }
}}

int main() {
    outer::foo();
}

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edit

It seems that gcc accepts this, as I first tested only on coliru, however clang rejects it