auto add = [](int x, int y) constexpr { return x + y; };

In the above example, the add lambda function is declared as constexpr, which means that it can be used in constant expressions and evaluated at compile time. This allows for more flexibility in using lambda functions in contexts that require compile-time evaluation.

One important thing to note is that, just like regular constexpr functions, constexpr lambdas have restrictions on their body, including but not limited to the inability to use non-const variables, assign to a non-const variable, or use runtime-only constructs such as dynamic memory allocation.

Using constexpr lambdas can be beneficial in scenarios where compile-time evaluation is desired, such as when working with template metaprogramming or constexpr contexts.

For more information on constexpr lambda functions in C++, you can refer to the C++ language standard (ISO/IEC 14882) or reputable online C++ resources.