Variant tries to mimic D's built-in rules for arithmetic 
conversions but:

import std.variant, std.stdio;
void main()
{
  auto v1 = Variant(4.5f);
  auto v2 = Variant(3.5f);
  writeln((v1+v2).type()); // double (should be float)
}

The reason is Variant doesn't try to convert to float in 
opArithmetic. I have submitted a pull request fixing this.