There are many places in the codebase where we use a pointer type (*, shared_ptr, unique_ptr, etc.) because of historical reasons (CBlockIndex*) or for polymorphism. In many such cases, we may want to avoid needing to handle the pointer being null, or make it clear in the function contract that we require a non-null pointer.

not_null<T> is the solution proposed by the C++ Core Guidelines (relevant section), and restricts pointers and smart pointers to hold non-null values.

Since I've learned about this, I've repeatedly ran into scenarios where I wish I could use it so as to make call semantics clearer and avoid misuse / unnecessary null handling.

There is a header-only implementation of not_null<T> here, however, there are two issues we need to solve:

1. The header-only implementation linked above requires the Expects and Ensures macros defined here, do we also import these or can we tweak our existing assertion macros to work?
2. There is also a strict_not_null<T>, which requires explicit construction. Do we need that or just not_null<T>?