A condition on a bound variable escapes its binder #1632

What you ran:

"2 * sum(ln(x - w), w in { w : w^3 + w + 1 = 0 })".ToEntity().Differentiate("x");

What it did: … provided not x - w = 0, in which w is free: the name the sum binds escaped it, and EvalNumerical on the derivative throws, since that condition cannot be decided. The product rule leaves 0 * sum(...), and 0 * f keeps f.DomainCondition, which conjoins every child's condition, a binder's body included: sum(1/(k - x), k, 1, 3).DomainCondition is not k - x = 0, and integral(1/(t - x), t, 0, 1).DomainCondition is not t - x = 0. Those two are expanded before it shows.

What it should have done: No condition mentions a bound name outside its binder. A binder's domain condition leaves out what its body says about the name it binds.