FullSimplify::cfn
Simplify::cfn
The value of ComplexityFunction does not give a real number when applied to expressions:
Simplify[1 + x + y(1 + x), ComplexityFunction -> -Length]This shows a valid value for the ComplexityFunction option in Simplify:
Simplify[1 + x + y(1 + x), ComplexityFunction -> (-Length[#]&)]