The C++ code is a simple list type, and also a bunch of tests for that type to confirm that it works as intended. The crucial trick here is that because static asserts are used, the test values are computed during compilation, such computations are forbidden (by the standard) from having any leaks or Undefined Behaviour. Anything allocated must be freed by the time the tests complete, and no language Undefined Behaviour is permitted.

The latter is pretty normal for other languages but is a big deal in C++ where UB is a constant plague. However many languages have either forbid or have strict limits on compile time heap allocation - after all that heap isn't going to still exist at runtime. Requiring that you free everything allocated fixes that hole and means you get free leak detection.

The compiler is doing a bunch of complicated stuff at compile time. In fact it's both a C++ compiler and a C++ interpreter.

static_assert is a compile-time check.

[] { ... }(); is an immediately executed lambda function (IIFE in javascript parlance).

list<int> list{} is a linked list of integers (double-linked, forward and backward). push_back() allocates more memory. pop() / clean() deallocates memory.

It is creating a bunch of objects, modifying them, then asserting their value all at compile time. For example the first example creates a list and asserts its size is 0. List normally allocated on heap, so I am guessing they have made changes in thay area in c++20 by making it Constexpr, which is a fancy way to say an expression can be known at compile time.