Learning about the ordering of templates with constraints
When a compiler encounters function calls or class template instantiations, it needs to figure out what overload (for a function) or specialization (for a class) is the best match. A function may be overloaded with different type constraints. Class templates can also be specialized with different type constraints. In order to decide which is the best match, the compiler needs to figure out which one is the most constrained and, at the same time, evaluates to
true after substituting all the template parameters. In order to figure this out, it performs the constraints normalization. This is the process of transforming the constraint expression into conjunctions and disjunctions of atomic constraints, as described at the end of the previous section.
An atomic constraint
A is said to subsume another atomic constraint
B. A constraint declaration
D1 whose constraints subsume the constraints of another declaration...