[Issue 19917] unions should require that all members are `= void` initialised

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19917

--- Comment #5 from kinke at gmx.net ---
Nah, I think LDC's way is fine, but it could be improved by *requiring*
(exactly) one field to be explicitly initialized - clear as day and not much
more syntax overhead:

union U {
  int i;
  float f = 1.0f;
  real r;
}

vs.

union U {
  int i = void;
  float f = 1.0f;
  real r = void;
}

and

union U {
  int i = 0;
  float f;
  real r;
}

vs.

union U {
  int i;
  float f = void;
  real r = void;
}

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