From 677e2f3d546d8e7f140d97cd8291a81a423e6451 Mon Sep 17 00:00:00 2001
From: Andrey Semashev <Lastique@users.noreply.github.com>
Date: Wed, 6 May 2020 18:56:38 +0300
Subject: [PATCH] Add cxxstd value 20 for C++20 (#592)

* Added cxxstd value 20 for C++20.

The -std=c++20 is supported by clang 10.

* Updated cxxstd=latest for clang 5.0 and newer.

clang 5.0 supports -std=c++17. clang 6.0 supports -std=c++2a.
clang 10.0 supports -std=c++20.

* Updated cxxstd=latest for gcc.

gcc 4.9 supports -std=c++14. gcc 6 supports -std=c++17. gcc 10 will support
-std=c++20.

* Added support for cxxstd=20 to toolsets that don't support C++20.
---
 src/tools/clang.jam                   | 5 ++++-
 src/tools/cray.jam                    | 2 +-
 src/tools/features/cxxstd-feature.jam | 4 ++--
 src/tools/gcc.jam                     | 5 ++++-
 src/tools/pgi.jam                     | 1 +
 src/tools/xlcpp.jam                   | 4 ++++
 7 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/tools/clang.jam b/tools/clang.jam
index bcb3837039..f1850db194 100644
--- a/tools/clang.jam
+++ b/tools/clang.jam
@@ -55,7 +55,10 @@ rule init-cxxstd-flags ( toolset : condition * : version )
     local dialects = [ feature.values <cxxstd-dialect> ] ;
     dialects = [ set.difference $(dialects) : gnu iso ] ;
     local std ;
-    if [ version-ge $(version) : 3.5 ] { std = 1z ; }
+    if [ version-ge $(version) : 10.0 ] { std = 20 ; }
+    else if [ version-ge $(version) : 6.0 ] { std = 2a ; }
+    else if [ version-ge $(version) : 5.0 ] { std = 17 ; }
+    else if [ version-ge $(version) : 3.5 ] { std = 1z ; }
     else if [ version-ge $(version) : 3.4 ] { std = 14 ; }
     else if [ version-ge $(version) : 3.3 ] { std = 11 ; }
     else { std = 03 ; }
diff --git a/tools/cray.jam b/tools/cray.jam
index 107b3dbbc5..d586af3af4 100644
--- a/tools/cray.jam
+++ b/tools/cray.jam
@@ -707,7 +707,7 @@ rule set-cxxstd-procedure ( targets * : sources * : properties * )
     local cxxstd = [ feature.get-values cxxstd : $(properties) ] ;
     local cray-cxxstd = ;
 
-    local unsupported-values = 2a ; # I don't know what '2a' means.
+    local unsupported-values = 2a 20 ; # I don't know what '2a' means.
     if $(cxxstd) && $(cxxstd) in $(unsupported-values)
     {
 
diff --git a/tools/features/cxxstd-feature.jam b/tools/features/cxxstd-feature.jam
index dfddf5bb78..3d7ba8d8cc 100644
--- a/tools/features/cxxstd-feature.jam
+++ b/tools/features/cxxstd-feature.jam
@@ -8,7 +8,7 @@ import feature ;
 #| tag::doc[]
 
 [[bbv2.builtin.features.cxxstd]]`cxxstd`::
-*Allowed values*: `98`, `03`, `0x`, `11`, `1y`, `14`, `1z`, `17`, `2a`,
+*Allowed values*: `98`, `03`, `0x`, `11`, `1y`, `14`, `1z`, `17`, `2a`, `20`,
 `latest`.
 +
 Specifies the version of the C++ Standard Language to build with. All the
@@ -28,7 +28,7 @@ is supported.
 |# # end::doc[]
 
 feature.feature cxxstd
-    : 98 03 0x 11 1y 14 1z 17 2a latest
+    : 98 03 0x 11 1y 14 1z 17 2a 20 latest
     : optional composite propagated ;
 
 #| tag::doc[]
diff --git a/tools/gcc.jam b/tools/gcc.jam
index 5ceb038fc5..f03336a5d7 100644
--- a/tools/gcc.jam
+++ b/tools/gcc.jam
@@ -499,8 +499,11 @@ local rule compile-link-flags ( * )
     local rule init-cxxstd-flags ( condition * : version )
     {
         local std ;
-        if [ version-ge $(version) : 8 ] { std = 2a ; }
+        if [ version-ge $(version) : 10 ] { std = 20 ; }
+        else if [ version-ge $(version) : 8 ] { std = 2a ; }
+        else if [ version-ge $(version) : 6 ] { std = 17 ; }
         else if [ version-ge $(version) : 5 ] { std = 1z ; }
+        else if [ version-ge $(version) : 4.9 ] { std = 14 ; }
         else if [ version-ge $(version) : 4.8 ] { std = 1y ; }
         else if [ version-ge $(version) : 4.7 ] { std = 11 ; }
         else if [ version-ge $(version) : 3.3 ] { std = 98 ; }
diff --git a/tools/pgi.jam b/tools/pgi.jam
index b59db4d4ec..6eed7759fb 100644
--- a/tools/pgi.jam
+++ b/tools/pgi.jam
@@ -57,6 +57,7 @@ flags pgi.compile.c++ OPTIONS <cxxstd>14 : -std=c++14 ;
 flags pgi.compile.c++ OPTIONS <cxxstd>1z : -std=c++17 ;
 flags pgi.compile.c++ OPTIONS <cxxstd>17 : -std=c++17 ;
 flags pgi.compile.c++ OPTIONS <cxxstd>2a : -std=c++17 ;
+flags pgi.compile.c++ OPTIONS <cxxstd>20 : -std=c++17 ;
 flags pgi.compile.c++ OPTIONS <cxxstd>latest : -std=c++17 ;
 
 flags pgi.compile OPTIONS <link>shared : -fpic ;
diff --git a/tools/xlcpp.jam b/tools/xlcpp.jam
index 1b66301cf1..a9e767cefb 100644
--- a/tools/xlcpp.jam
+++ b/tools/xlcpp.jam
@@ -112,6 +112,10 @@ flags xlcpp.compile.c++ OPTIONS <cxxstd>0x : -std=c++11 ;
 flags xlcpp.compile.c++ OPTIONS <cxxstd>11 : -std=c++11 ;
 flags xlcpp.compile.c++ OPTIONS <cxxstd>1y : -std=c++1y ;
 flags xlcpp.compile.c++ OPTIONS <cxxstd>14 : -std=c++1y ;
+flags xlcpp.compile.c++ OPTIONS <cxxstd>1z : -std=c++1y ;
+flags xlcpp.compile.c++ OPTIONS <cxxstd>17 : -std=c++1y ;
+flags xlcpp.compile.c++ OPTIONS <cxxstd>2a : -std=c++1y ;
+flags xlcpp.compile.c++ OPTIONS <cxxstd>20 : -std=c++1y ;
 flags xlcpp.compile.c++ OPTIONS <cxxstd>latest : -std=c++1y ;
 
 flags xlcpp.compile OPTIONS <cflags> ;