c++ - Workaround for lvalue-to-rvalue conversion of the inactive member of a union within relaxed constant expressions -

The larval-to-rivalview conversion is not a continuous expression for converting a union into a union. given:

  template   
 and  constexpr  function  foo :  
  in the template's & lt; Square t, square u & gt; Constexpr auto foo () {A & lt; T, U & gt; On{}); Return a.u_; }   
 The following program:  
  int main () {constexpr auto test = foo & lt; Int, double> (); Return 0; }   
 Fails with error message:  
  Error: constexpr variable 'test' constexpr auto test = foo & lt start by a continuous expression should be done; Integer, double & gt; (); ^ ~~~~~~~~~~~~~~~~~~ NOTE: The member's member 'u_' is not allowed to be read with active member 'T_', in a continuous expression a.u_ ; ^ Note: In the call 'fu ()' constexpr auto test = foo & lt; Int, double> (); ^ 1 error occurred.   
 Is this behavior achievable in C ++ 14 continuous expression? .  reinterpret_cast  is not allowed either  
 Inspiration:. I am trying to make the Google testing floating point compare utility constexpr   
 
 
 No, C ++ 14 There is no way of typing in continuous expression. There is no way to do this outside of continuous expression; The code you show will have undefined behavior in runtime.  
 For example, why not normally refuse, consider this:  
  union y {int * p; Size_t n; }; Int a; Constexpr UU = {& amp; One}; Static_assert (u.n! = 0x400e158, "One is at my favorite address");   
 If this permission was granted, the implementation would be required to determine the runtime address of  a  during the compilation.