Added std::is_constant_evaluated.

CPP20.md

@@ -23,6 +23,7 @@ C++20 includes the following new library features:
 - [std::span](#stdspan)
 - [bit operations](#bit-operations)
 - [math constants](#math-constants)
+- [std::is_constant_evaluated](#stdis_constant_evaluated)
 
 ## C++20 Language Features
 
@@ -426,6 +427,17 @@ std::numbers::pi; // 3.14159...
 std::numbers::e; // 2.71828...
 ```
 
+### std::is_constant_evaluated
+Predicate function which is truthy when it is called in a compile-time context.
+```c++
+constexpr bool is_compile_time() {
+    return std::is_constant_evaluated();
+}
+
+constexpr bool a = is_compile_time(); // true
+bool b = is_compile_time(); // false
+```
+
 ## Acknowledgements
 * [cppreference](http://en.cppreference.com/w/cpp) - especially useful for finding examples and documentation of new library features.
 * [C++ Rvalue References Explained](http://thbecker.net/articles/rvalue_references/section_01.html) - a great introduction I used to understand rvalue references, perfect forwarding, and move semantics.