core: Don't panic if "required replace" path doesn't resolve to a value

terraform/eval_diff.go

@@ -214,11 +214,13 @@ func (n *EvalDiff) Eval(ctx EvalContext) (interface{}, error) {
 				// the same type, but we'll allow it for robustness.
 				reqRep = append(reqRep, path)
 			}
+			if priorChangedVal != cty.NilVal {
 				eqV := plannedChangedVal.Equals(priorChangedVal)
 				if !eqV.IsKnown() || eqV.False() {
 					reqRep = append(reqRep, path)
 				}
 			}
+		}
 		if diags.HasErrors() {
 			return nil, diags.Err()
 		}