bug#41279: 26.3; Calc: sum(gcd(0,n),n,-1,-1) evaluates to -1

[Mattias Engdegård]

Thank you for reporting this. It seems that Calc first simplifies gcd(0,n) to n 
and then computes sum(n,n,-1,-1) = -1.
We could either make gcd(0,x)=gcd(x,0)=|x|, or try preventing sum from 
simplifying its operand first, or both.
For the smallest possible change, what about the hack below?

--- a/lisp/calc/calc-comb.el
+++ b/lisp/calc/calc-comb.el
@@ -241,8 +241,8 @@ calcFunc-gcd
         (calcFunc-gcd (math-neg a) b))
        ((Math-looks-negp b)
         (calcFunc-gcd a (math-neg b)))
-       ((Math-zerop a) b)
-       ((Math-zerop b) a)
+       ((Math-zerop a) (math-abs b))
+       ((Math-zerop b) (math-abs a))
        ((and (Math-ratp a)
              (Math-ratp b))
         (math-make-frac (math-gcd (if (eq (car-safe a) 'frac) (nth 1 a) a)