Up to and including C++17 pi is not a constant introduced into the language, and it’s a pain in the neck.
I’m fortunate in that I use boost and they define pi with a sufficiently large number of decimal places for even a 128 bit long double.
If you don’t use Boost then hardcode it yourself. Defining it with a trigonometric function is tempting but if you do that you can’t then make it a constexpr. The accuracy of the trigonometric functions is also not guaranteed by any standard I know of (cf. std::sqrt), so really you are on dangerous ground indeed relying on such a function.
There is a way of getting a constexpr value for pi using metaprogramming: see http://timmurphy.org/2013/06/27/template-metaprogramming-in-c/
From C++20 some good news. There is a defininition for pi. C++20 adds some mathematical constants in <numbers>. For example std::numbers::pi is a double type.
Reference: https://en.cppreference.com/w/cpp/numeric/constants