guides/upgrade/v1912-Developer-Upgrade-Guide.md

@@ -15,8 +15,51 @@
 
 ## Changes in definition
 
+- Replaced previous use of `contiguous<T>()` as a global function with
+  `is_contiguous<T>' as a traits class. For some advanced user code,
+  this can be a breaking change, but greatly improves the ability to
+  define general traits and to inherit from them.
+
+  Previously something like the following was required:
+  ```
+  template<>
+  bool contiguous<FixedList<bool, 2>>() { return true; }
+
+  template<>
+  bool contiguous<FixedList<int_8, 2>>() { return true; }
+  ...
+  ```
+  This repetitious and error-prone definition is now replaced with a
+  more direct and succinct form:
+  ```
+  template<class T, unsigned N>
+  struct is_contiguous<FixedList<T, N>> : struct is_contiguous<T> {};
+  ```
+
+  An future benefit of the new structure is that it provides a static
+  constexpr `value`, which allows its use in template expressions.
+
+  The change in definition is accompanied by a slight name change from
+  `contiguous` to `is_contiguous`, which should assist in identifying
+  upgraded code.
+
+  When defining the contiguous characteristics, the `is_contiguous_label`
+  and the `is_contiguous_scalar` forms may also be relevant.
+  These are special traits for handling data of homogeneous components
+  of the respective types.
+
+
 ## Changes in behaviour
 
+- The addition of an optional match type (LITERAL vs REGEX) for
+  dictionary::subOrEmptyDict()
+  - old: `subOrEmptyDict(key, bool=false)`
+  - new: `subOrEmptyDict(key, keyType::option=keyType::REGEX, bool=false)`
+
+  This presents a minor breaking change for code that used the bool parameter
+  to specify mandatory vs. non-mandatory existence.
+
+
 ## Namespace changes
 
 [v1912-notes]: https://www.openfoam.com/releases/openfoam-v1912/