How to declare constexpr c-string member? [duplicate]

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• Undefined reference to static variable c++ 2 answers
• Undefined reference to static constexpr char[] 6 answers

How to declare constexpr c-string member ? Non-member constexpr c-string can be declared this way:

constexpr char id_[] = "aaa";

so I have though that I can declare it this way:

struct T
{
    const static constexpr char id_[] = "aaa";
};

but I get undefined reference error.