What means "obey ODR" in case of inline and constexpr function?

I just read that constexpr and inline functions obey one-definition rule, but they definition must be identical. So I try it:

inline void foo() {
    return;
}

inline void foo() {
    return;
}

int main() {
    foo();
};

error: redefinition of 'void foo()',

and

constexpr int foo() {
    return 1;
}

constexpr int foo() {
    return 1;
}

int main() {
    constexpr x = foo();
}; 

error: redefinition of 'constexpr int foo()'

So what exactly means that, constexpr and inline function can obey ODR?