bug#41925: 28.0.50; Error using calc-degrees-mode with calc-base-units

[Mattias Engdegård]

It looks like Calc explicitly switches to radians mode during simplification 
with units:

(defun math-from-radians (a)   ; [N N]
  (cond ((and (not math-simplifying-units)
              (eq calc-angle-mode 'deg))
         (if (math-constp a)
             (math-div a (math-pi-over-180))
           (list 'calcFunc-deg a)))
        ((eq calc-angle-mode 'hms)
         (math-to-hms a 'rad))
        (t a)))

Presumably, this is done because the unit calculations somehow expect 
everything to be done in radians. I'm not sure how the machinery works, but to 
hazard a guess, suppose that

 sin(30 deg)

would unit-simplify into

 sin(30*(π/180))

then that would only give the right result if sin(x) only accepted radians no 
matter the current angle mode during this simplification process.

Obviously I didn't write Calc, and am not sure how to solve it without blowing 
something else apart. Perhaps you want to give it a go?

It also seems unclear how units and angle mode are supposed to interact. In 
degrees mode, arctan(1) evaluates to a dimensionless 45, not 45 deg.