I recently posed a challenge about computing a function of \$\pi\$. It was closed for being unclear and received a number of criticisms. One of the main ones I think was that banning built-ins for \$\pi\$ is not clear as there are always other equivalent built-ins one hasn't thought of.
Looking at old questions I see that banning built-ins for \$\pi\$ is quite common however. Take for example:
Output with the same number of digits of pi as the length of the code : "Built-ins to calculate pi cannot be used. This includes pi constants (e.g. math.pi in Python) and built-in functions that return either the value of pi, or the n-th digit of pi. This means that an algorithm which calculates pi, or the n-th digit of pi must be implemented in your answer."
Monte Carlo estimator of Pi : "No trig functions or Pi constants, must be a Monte Carlo approach."
Calculate 500 digits of pi : "It cannot include "pi", "math.pi" or similar pi constants, nor may it call a library function to calculate pi."
and so on.
I have now changed the question to make it easier.