Update documentation and few more defines

docs/source/yosys_internals/extending_yosys/contributing.rst

@@ -286,7 +286,7 @@ have incorrect results in unusual situations.
 Coding style
 ~~~~~~~~~~~~
 
-Yosys is written in C++17.
+Yosys is written in C++20.
 
 In general Yosys uses ``int`` instead of ``size_t``. To avoid compiler warnings
 for implicit type casts, always use ``GetSize(foobar)`` instead of

docs/source/yosys_internals/index.rst

@@ -25,7 +25,7 @@ wide range of real-world designs, including the `OpenRISC 1200 CPU`_, the
 
 .. _k68 CPU: http://opencores.org/projects/k68
 
-Yosys is written in C++, targeting C++17 at minimum. This chapter describes some
+Yosys is written in C++, targeting C++20 at minimum. This chapter describes some
 of the fundamental Yosys data structures. For the sake of simplicity the C++
 type names used in the Yosys implementation are used in this chapter, even
 though the chapter only explains the conceptual idea behind it and can be used