I did that, but then adding 0_degC + 0_degC gave 273.15_degC. I think that would be surprising to many people. In reality I don't remember ever having to deal with physics formulas that used °C instead of Kelvins, so the only time I need to use °C is when presenting the temperature in the UI:

    temperature.in<si::Celsius>()

Duh. I was thinking too much on the syntax side and not thinking about the physical world; which is ironic considering the discussion topic.

Adding temperatures is not done usually. Mixing substances adds thermal energy. There's no reason I can hink of why allowing plus operator on temperatures is a good idea. You can add temperature differentials, but it is plain addition so long as units are in agreement. And 0C differential is 0F differential, no offsets in conversions.