You don't get a warning because the program is doing exactly what you asked it to do. You said a C with x = 0 is equal to 'true'. And you said a D with f = 0.0

You don't get a warning because the program is doing exactly what you asked it to do. You said a C with x = 0 is equal to 'true'. And you said a D with f = 0.0 is equal to 'true'. By transitivity of equality ...