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  1. struct C {
  2. constexpr C(int i) : i(i) {}
  3. int i;
  4. };
  5.  
  6.  
  7. constexpr C plus2(const C& c) { return C{c.i + 2}; }
  8.  
  9. int main()
  10. {
  11. const C i(0);
  12. constexpr C j = plus2(i);
  13. }
  14.  
Compilation error #stdin compilation error #stdout 0s 3408KB
stdin
Standard input is empty
compilation info
prog.cpp: In function 'int main()':
prog.cpp:12:28:   in constexpr expansion of 'plus2(i)'
prog.cpp:12:28: error: the value of 'i' is not usable in a constant expression
     constexpr C j = plus2(i);
                            ^
prog.cpp:11:13: note: 'i' was not declared 'constexpr'
     const C i(0);
             ^
stdout
Standard output is empty