Explore a proposed C++ language feature called Parametric Expressions in this conference talk from C++Now 2019. Delve into the concept of hygienic macros on expressions that occur during semantic analysis and template instantiation, understanding how they function as expressions without actual function calls or types. Learn how Parametric Expressions can lead to more concise interfaces, hide complex template code, and enable lazy evaluation. Discover the potential for faster build times due to the absence of features like overloading, function type instantiation, and SFINAE. Begin with an introductory explanation suitable for newcomers, followed by progressively more advanced use cases beneficial to library authors. Engage in audience participation and discussion while exploring techniques for mixing compile-time and run-time computations. Gain insights from Jason Rice, a web applications programmer with expertise in C++ metaprogramming and contributor to Boost.Hana.