It is generally a performance bug to not make them static, as people assume constexpr means static duration. Objects get re-initialized each function call. This also can lead to UB if returning a constexpr object assuming it has a static lifetime.

clang-tidy has an open PR with the appropriate check that has been open for a while. Can take the implementation to create our own check and then replace it when shipped. https://github.com/llvm/llvm-project/pull/147809/changes. See this issue for more details about the check + fix: https://github.com/llvm/llvm-project/issues/146296

NOTE: Can't do this for constexpr or consteval functions until C++23. They don't support static in the function bodies. See https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2647r1.html