update c++ version to 20

rules.org

@@ -66,7 +66,7 @@ This document is derived from the original ATLAS C++ coding standard,
 derived from work done by the CERN ``Project support team''
 and SPIDER project, as documented in [[http://pst.web.cern.ch/PST/HandBookWorkBook/Handbook/Programming/CodingStandard/c++standard.pdf][CERN-UCO/1999/207]] [fn:PST99].
 These previous guidelines have been significantly revised
-to take into account the evolution of the C++ language [fn:Cxx17],
+to take into account the evolution of the C++ language [fn:Cxx20],
 current practices in ATLAS, and the experience gained
 over the past decade.
 
@@ -2598,12 +2598,12 @@ float ip_cut = 0.1 * Gaudi::Units::cm;
 
 ** Portability
 
- - *All code must comply with the 2017 version of the ISO C++ standard (C++17)*.  [<<standard-cxx>>]
+ - *All code must comply with the 2020 version of the ISO C++ standard (C++20)*.  [<<standard-cxx>>]
 
    A draft of the standard which is essentially identical to the final version
-   may be found at [fn:Cxx17].
+   may be found at [fn:Cxx20].
 
-   At some point, compatibility with C++20 will also be required.
+   At some point, compatibility with C++23 will also be required.
 
 
  - *Make non-portable code easy to find and replace.*  [<<limit-non-portable-code>>]
@@ -3038,6 +3038,7 @@ The comment includes the fact that it is the perpendicular distance.
 * Changes
 
 ** Version 0.8
+ - Updated for C++20.
  - Clarify that non-ascii characters should not be used in identifier names.
  - Clarify that variable-length argument lists of varadic template functions
    are ok.
@@ -3092,7 +3093,7 @@ The comment includes the fact that it is the perpendicular distance.
 [fn:GOF] E. Gamma, R. Helm, R. Johnson, and J. Vlissides, /Design Patterns, Elements of Reusable Object-Oriented Software/, Addison-Wesley.
 [fn:Sutter02] [[http://www.gotw.ca/publications/mill22.htm][H. Sutter, /C++ Users Journal/, *20* (7), 2002.]]
 [fn:Krzemienski14] [[http://akrzemi1.wordpress.com/2014/04/24/noexcept-what-for/][A. Krzemie\nacute{}ski, /noexcept --- what for?/, 2014.]]
-[fn:Cxx17] [[https://github.com/cplusplus/draft/blob/master/papers/n4659.pdf][ /Standard for the Programming Language C++/, n4659.]]
+[fn:Cxx20] [[https://github.com/cplusplus/draft/releases/download/n4868/n4868.pdf][/Standard for the Programming Language C++/, n4868.]]
 [fn:Warnings] [[https://twiki.cern.ch/twiki/bin/view/AtlasComputing/FaqCompileTimeWarnings][FaqCompileTimeWarnings ATLAS wiki page.]]
 [fn:Doxygen] [[https://twiki.cern.ch/twiki/bin/view/AtlasComputing/DoxygenDocumentation][DoxygenDocumentation ATLAS wiki page.]]