C++17 STL Cookbook

constexpr (optional)

If we mark the lambda expression explicitly as constexpr, the compiler will error out if it does not satisfy the criteria of constexpr functions. The advantage of constexpr functions and lambda expressions is that the compiler can evaluate their result at compile time if they are called with compile-time constant parameters. This leads to less code in the binary later.

If we do not explicitly declare the lambda expression to be constexpr but it fits the requirements for that, it will be implicitly constexpr anyway. If we want a lambda expression to be constexpr, it helps to be explicit because the compiler will then help us by erroring out if we did it wrong.