Thanks for answering!  So it seems like your answer is "the standard units library should not support converting temperature differences from Celsius to Fahrenheit, because these temperature differences should not be representable in the first place".  It's certainly not the approach I'd prefer, but I guess it's one point of view.

But that's not the end of the story --- let's go further and imagine that this design gets accepted.  Imagine a climate scientist has a temperature increase of 1.5 degrees Celsius, and wants to express it in Fahrenheit.  What will happen?  Will the library prevent this?

It seems to me that the likeliest outcome is that they'll create a temperature of 1.5 degrees Celsius, and request conversion to a temperature in Fahrenheit, at which point the library happily provides a result of 34.7 degrees F, rather than the 2.7 degrees F I imagine they would have wanted.

Your original post provided a great service --- a concrete example where the current units library proposal would return a grossly incorrect result for code that looks correct.  We can fix this if we avoid making it easy to create a quantity in situations where a quantity point is needed.  By contrast, it seems to me that your proposal suffers from the exact same defect --- good-looking code that produces grossly incorrect results --- except that in this case, I'm at a loss for how we could fix it.

Cheers,

Chip