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#include <array>
 
template< std::size_t len >
struct str_wrap : std::array< char, len > {
    typedef char (& arr_t)[ len ];
    operator arr_t () { return reinterpret_cast< char (&)[ len ] >( * this->data() ); }
    operator arr_t const () const { return reinterpret_cast< char const (&)[ len ] >( * this->data() ); }
};
 
template< std::size_t lhs_len, std::size_t rhs_len >
constexpr str_wrap< lhs_len + rhs_len - 1 > const cat( char const (&lhs)[ lhs_len ], char const (&rhs)[ rhs_len ] ) {
        return {};
}
 
template< std::size_t lhs_len, std::size_t rhs_len >
constexpr str_wrap< lhs_len + rhs_len - 1 > const cat( str_wrap< lhs_len >, char const (&rhs)[ rhs_len ] ) {
        return {};
}
 
template< std::size_t lhs_len, std::size_t rhs_len >
constexpr str_wrap< lhs_len + rhs_len - 1 > const cat( char const (&lhs)[ lhs_len ], str_wrap< rhs_len > ) {
        return {};
}
 
int main() {
cat( "a", cat( "bcd", "ef" ) );
}
 
 

prog.cpp: In function 'const str_wrap<((lhs_len + rhs_len) - 1)> cat(const char (&)[lhs_len], const char (&)[rhs_len]) [with unsigned int lhs_len = 4u, unsigned int rhs_len = 3u]':
prog.cpp:26:28:   instantiated from here
prog.cpp:11:51: error: parameter 'const char (& lhs)[4]' is not of literal type
prog.cpp: In function 'const str_wrap<((lhs_len + rhs_len) - 1)> cat(const char (&)[lhs_len], str_wrap<rhs_len>) [with unsigned int lhs_len = 2u, unsigned int rhs_len = 6u]':
prog.cpp:26:30:   instantiated from here
prog.cpp:21:51: error: parameter 'const char (& lhs)[2]' is not of literal type

Standard output is empty