--- /dev/null
+Changed: Every deal.II header file includes the deal.II header
+`deal.II/base/config.h` that contains some basic configuration
+information such as what deal.II version is being used, whether
+deal.II was configured with Trilinos/PETSc/... support, etc. In turn,
+`config.h` included `deal.II/base/types.h` and
+`deal.II/base/numbers.h`, making the contents of these latter two
+files available throughout deal.II.
+
+For conceptual reasons, for example to support C++20 modules in the
+long run, the `deal.II/base/config.h` file must stand separate from
+the rest of the deal.II header files. As a consequence, it can no
+longer have includes for other deal.II header files (namely,
+`deal.II/base/types.h` and `deal.II/base/numbers.h` as mentioned
+above), and no longer does so now. If your code requires knowledge of
+the contents of `deal.II/base/types.h` and `deal.II/base/numbers.h`,
+you will have to explicitly include these files like you do with any
+other deal.II header file already.
+
+<br>
+(Wolfgang Bangerth, 2024/01/16)