template<
    std::size_t M>
static_string&
operator+=(
    static_string< M, CharT, Traits > const& str);
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static_string&
operator+=(
    CharT ch);
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static_string&
operator+=(
    CharT const* s);
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static_string&
operator+=(
    std::initializer_list< CharT > init);
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static_string&
operator+=(
    string_view_type const& str);
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