smichr/sympy — comment on commit a00f5e8c5ffec22f2a15acc2b105478cca9a6619
File: sympy/concrete/summations.py line 206
By smichr:

I don't think so: the equality is handled in subs; the rest of the routine is just rnning through the arguments (expr and limits) with the constraint that old not match a limit variable. So that all simplifies down to 'if old is not in the variables fallback to doing an all-arg subs' which is what happens when a _subs returns None.