Update technical todo

Add a section mentioning C++20 and the new constexpr
std::string and std::vector that we can use.

Documentation/development/technical_todo.md

@@ -29,6 +29,9 @@ the version switch exists and can be tidied.
 
 ## C++14 {#todo_cpp_14}
 
+This C++ standard version is already used by KiCad on the development branch, but much code
+that pre-dates this version can be tidied.
+
 Compiler support:
 
 * GCC: complete by GCC 5: [GCC C++14 support](https://gcc.gnu.org/projects/cxx-status.html#cxx14).
@@ -48,6 +51,12 @@ Provides:
   can replace `boost::filesystem` and the Boost dependency can be dropped entirely
   from the CMake files.
 
+
+## C++20 {#todo_cpp_20}
+
+* `constexpr std::string` and `constexpr std::vector` can be used to initialize items:
+    * File extensions in `wildcards_and_files_ext.h`
+
 # Compilers {#compilers}
 
 Some compilers have bugs or limitations to C++ standard support.

pcbnew/dialogs/panel_fp_lib_table.cpp

@@ -91,10 +91,11 @@ enum {
 static const std::map<int, supportedFileType>& fileTypes()
 {
     /*
-    * This is wrapped inside a function to prevent a static initialization order fiasco with the file extension
+     * TODO(C++20): Clean this up
-    * variables. Once C++20 is allowed in KiCad code, those file extensions can be made constexpr and this can
+     * This is wrapped inside a function to prevent a static initialization order fiasco with the file extension
-    * be removed from a function call and placed in the file normally.
+     * variables. Once C++20 is allowed in KiCad code, those file extensions can be made constexpr and this can
-    */
+     * be removed from a function call and placed in the file normally.
+     */
     static const std::map<int, supportedFileType> fileTypes =
     {
         { ID_PANEL_FPLIB_ADD_KICADMOD,    { "KiCad (folder with .kicad_mod files)", "",                               KiCadFootprintFileExtension,      false, IO_MGR::KICAD_SEXP } },