build in C++17 mode by default

* configure.ac: Activate C++17, and replace --enable-c++17 by
--enable-c++20.
* NEWS: Mention the news.
* .gitlab-ci.yml: Use C++20 for the former C++17 builds.
* HACKING, README, doc/org/compile.org, doc/org/concepts.org,
doc/org/index.org, doc/org/install.org, doc/org/tut.org,
doc/org/upgrade2.org, spot/misc/escape.hh: Adjust mentions
of C++14.

.gitlab-ci.yml

@@ -99,7 +99,7 @@ arch-clang:
   image: registry.lrde.epita.fr/spot-arch
   script:
     - autoreconf -vfi
-    - ./configure --prefix ~/install_dir CC='clang -Qunused-arguments' CXX='clang++ -Qunused-arguments' --enable-devel --enable-c++17 --enable-doxygen
+    - ./configure --prefix ~/install_dir CC='clang -Qunused-arguments' CXX='clang++ -Qunused-arguments' --enable-devel --enable-c++20 --enable-doxygen
     - make
     - make distcheck
   artifacts:
@@ -117,9 +117,9 @@ arch-gcc-glibcxxdebug:
   image: registry.lrde.epita.fr/spot-arch
   script:
     - autoreconf -vfi
-    - ./configure --enable-devel --enable-c++17 --enable-glibcxx-debug
+    - ./configure --enable-devel --enable-c++20 --enable-glibcxx-debug
     - make
-    - make distcheck DISTCHECK_CONFIGURE_FLAGS='--enable-devel --enable-c++17 --enable-glibcxx-debug'
+    - make distcheck DISTCHECK_CONFIGURE_FLAGS='--enable-devel --enable-c++20 --enable-glibcxx-debug'
   artifacts:
     when: on_failure
     paths:

HACKING

@@ -24,7 +24,7 @@ since the generated files they produce are distributed.)
   GNU Autoconf >= 2.61
   GNU Automake >= 1.11
   GNU Libtool >= 2.4
-  GNU Flex (the version seems to matters, we used 2.5.35)
+  GNU Flex >= 2.5.35
   GNU Bison >= 3.0
   GNU Emacs (preferably >= 24 but it may work with older versions)
   org-mode >= 9.1 (the version that comes bundled with your emacs
@@ -32,9 +32,9 @@ since the generated files they produce are distributed.)
     a separate and more recent org-mode package, or you can
     simply upgrade from ELPA).
   Groff (a.k.a. GNU troff) >= 1.20
-  SWIG >= 3.0.2 (for its better C++11/C++14 support)
+  SWIG >= 3.0.2, preferably >= 4.0.1 (for its better modern C++ support)
-    Note that if you use Macports you should consider to install the
+    If you use Macports you consider installing the swig-python package
-    swig-python package in addition to the swig package.
+    in addition to the swig package.
   Doxygen >= 1.4.0
   R   used by some examples in the documentation
   Perl, with its Gettext module (it might be called something like
@@ -311,14 +311,22 @@ forget guards, and we do not forget to rename them when a file is
 copied into another one.
 
 
-C++14
+C++17
 -----
 
-  Spot uses some C++14 features, and therefore requires a C++14
+  Spot uses some C++17 features, and therefore requires a C++17
-  compiler.  g++ 5.x or clang++ 3.4 should be enough.
+  compiler.  g++ 7.x or clang++ 5 should be enough.
+
+  Note that the current stable release, 2.9.x, is still using C++14,
+  so avoid rewriting the code base into C++17 just because it's nicer,
+  as it will make harder to backport bug fixes.  Feel free to
+  introduce C++17 feature in new code, or in algorithms that are
+  largely rewritten.
+
+  We currently avoid C++20 features until C++20 compiler are widely
+  available.  (FYI: we switched to C++11 in 2015, to C++14 in 2017,
+  and to C++17 in 2020.)
 
-  We currently avoid C++17 features until C++17 compiler are widely
-  available.
 
 Encoding
 --------

NEWS

@@ -1,5 +1,13 @@
 New in spot 2.9.1.dev  (not yet released)
 
+  Build:
+
+  - Spot is now built in C++17 mode, so you need at least GCC 7 or
+    clang 5 to compile it.  The current versions of all major linux
+    distributions ship with at least GCC 7.4, so this should not be a
+    problem.  There is also an --enable-c++20 option to configure in
+    case you want to force a build of Spot in C++20 mode.
+
   Library:
 
   - Historically, Spot labels automata by Boolean formulas over

README

@@ -2,7 +2,7 @@ Overview
 ========
 
 Spot is a model-checking toolkit comprising:
-  - a C++14 library with data-structures and algorithms for working
+  - a C++17 library with data-structures and algorithms for working
     with linear-time temporal logical formulas and ω-automata with
     any acceptance condition.
   - a set of command-line tools for easy access to those algorithms,
@@ -73,8 +73,8 @@ Installation
 Requirements
 ------------
 
-Spot requires a C++14-compliant compiler.  G++ 5.x or later, as well
+Spot requires a C++17-compliant compiler.  G++ 7.x or later, as well
-as Clang++ 3.5 or later should work.
+as Clang++ 5.0 or later should work.
 
 Spot expects a complete installation of Python (version 3.3 or later).
 Especially, Python's headers files should be installed.  If you don't

configure.ac

@@ -47,11 +47,11 @@ adl_CHECK_BISON
 # Decrease verbosity when passing argument V=0
 AM_SILENT_RULES([no])
 
-# Option to activate C++17
+# Option to activate C++20
-AC_ARG_ENABLE([c++17],
+AC_ARG_ENABLE([c++20],
-              [AC_HELP_STRING([--enable-c++17],
+              [AC_HELP_STRING([--enable-c++20],
-                              [Compile in C++17 mode.])],
+                              [Compile in C++20 mode.])],
-              [enable_17=$enableval], [enable_17=no])
+              [enable_20=$enableval], [enable_20=no])
 
 AC_ARG_ENABLE([doxygen],
               [AC_HELP_STRING([--enable-doxygen]),
@@ -93,8 +93,8 @@ AX_CHECK_COMPILE_FLAG([-Werror -fvisibility=hidden],
     [CXXFLAGS="$CXXFLAGS -fvisibility-inlines-hidden"])])
 CXXFLAGS="$CXXFLAGS -DSPOT_BUILD"
 
-# Turn on C++14 support
+# Turn on C++17 support
-m4_define([_AX_CXX_COMPILE_STDCXX_14_testbody],
+m4_define([_AX_CXX_COMPILE_STDCXX_17_testbody],
 [AC_LANG_SOURCE([#include <memory>
 #include <string>
 #include <chrono>  // used to fail in C++11 with some installation of clang
@@ -118,28 +118,31 @@ m4_define([_AX_CXX_COMPILE_STDCXX_14_testbody],
     check_type&& cr = static_cast<check_type&&>(c);
 
     auto d = a;
+
+    int aa[[2]] = {1,2};
+    auto [[x,y]] = aa;
 ])])
 
-if test x"${enable_17}" = xyes; then
+if test x"${enable_20}" = xyes; then
+  for f in -std=c++20 '-std=c++20 -stdlib=libc++' -std=c++2a
+  do
+    AX_CHECK_COMPILE_FLAG([$f], [CXXFLAGS="$CXXFLAGS $f" stdpass=true], [], [],
+                          [_AX_CXX_COMPILE_STDCXX_17_testbody])
+    ${stdpass-false} && break
+  done
+  if ! "${stdpass-false}"; then
+    AC_ERROR([unable to turn on C++20 mode with this compiler])
+  fi
+else
   for f in -std=c++17 '-std=c++17 -stdlib=libc++' -std=c++1z
   do
     AX_CHECK_COMPILE_FLAG([$f], [CXXFLAGS="$CXXFLAGS $f" stdpass=true], [], [],
-                          [_AX_CXX_COMPILE_STDCXX_14_testbody])
+                          [_AX_CXX_COMPILE_STDCXX_17_testbody])
     ${stdpass-false} && break
   done
   if ! "${stdpass-false}"; then
     AC_ERROR([unable to turn on C++17 mode with this compiler])
   fi
-else
-  for f in -std=c++14 '-std=c++14 -stdlib=libc++' -std=c++1y
-  do
-    AX_CHECK_COMPILE_FLAG([$f], [CXXFLAGS="$CXXFLAGS $f" stdpass=true], [], [],
-                          [_AX_CXX_COMPILE_STDCXX_14_testbody])
-    ${stdpass-false} && break
-  done
-  if ! "${stdpass-false}"; then
-    AC_ERROR([unable to turn on C++14 mode with this compiler])
-  fi
 fi
 
 AX_CHECK_BUDDY

doc/org/compile.org

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 #+TITLE: Compiling against Spot
-#+DESCRIPTION: How to compile C++14 programs using Spot
+#+DESCRIPTION: How to compile C++17 programs using Spot
 #+INCLUDE: setup.org
 #+HTML_LINK_UP: tut.html
 #+PROPERTY: header-args:C+++ :results verbatim :exports both
@@ -31,19 +31,19 @@ greetings and the Spot version:
 
 #+RESULTS: hello-word
 : Hello world!
-: This is Spot 2.7.2.dev.
+: This is Spot 2.9.1.dev.
 
 
 To successfully compile this example program, we need a C++ compiler,
 obvisously.  On this page, we are going to assume that you use =g++=
-(version 4.8 or later), but other compilers like =clang++= share the
+(version 7 or later), but other compilers like =clang++= share the
 same user interface.  To successfully build the =hello= program, we
 might need to tell the compiler several things:
 
-1. The language that we use is C++14 (or optionally C++17).  This
+1. The language that we use is C++17 (or optionally C++20).  This
-   usually requires passing an option like =-std=c++14=.  Note that
+   usually requires passing an option like =-std=c++17=.  Note that
-   with version 6 of =g++= the default is now to compile C++14, so
+   with version 11 of =g++= the default will be to compile C++17, so
-   this option is not necessary.
+   this option will not be necessary.
 2. The C++ preprocessor should be able to find =spot/misc/version.hh=.
    This might require appending another directory to the include
    search path with =-I location=.

doc/org/concepts.org

@@ -1025,7 +1025,7 @@ layers.
 
   - =libbddx= is a customized version of [[https://sourceforge.net/projects/buddy/][the BuDDy library]], for
     manipulating [[#bdd][BDDs]].
-  - =libspot= is the main library, containing a C++14 implementation of all the
+  - =libspot= is the main library, containing a C++17 implementation of all the
     data structures and algorithms.  This depends on =libddx=.
   - =libspotgen= is an auxiliary library that contains functions to
     generate families of automata, useful for benchmarking and testing
@@ -1037,7 +1037,7 @@ layers.
     SpinS or a patched version of DiVinE, but you have to install
     those third-party tools first.  See [[https://gitlab.lrde.epita.fr/spot/spot/blob/next/tests/ltsmin/README][=tests/ltsmin/README=]]
     for details.
-  - In addition to the C++14 API, we also provide Python bindings for
+  - In addition to the C++17 API, we also provide Python bindings for
     =libspotgen=, =libspotltsmin=, =libbddx=, and most of =libspot=.
     These are available by importing =spot.gen=, =spot.ltsmin=, =bdd=,
     and =spot=.  Those Python bindings also includes some additional

doc/org/index.org

@@ -1,11 +1,11 @@
 # -*- coding: utf-8 -*-
 #+TITLE: Spot: a platform for LTL and ω-automata manipulation
 #+DESCRIPTION: Spot is a library and tool suite for LTL and ω-automata
-#+KEYWORDS: Spot,C++14,library,platform,framework,tool-suite,LTL,PSL,omega-automata
+#+KEYWORDS: Spot,C++17,library,platform,framework,tool-suite,LTL,PSL,omega-automata
 #+INCLUDE: setup.org
 #+HTML_HEAD_EXTRA: <style>#org-div-home-and-up { display: none; }</style>
 
-Spot is a C++14 library for LTL, ω-automata manipulation and model
+Spot is a C++17 library for LTL, ω-automata manipulation and model
 checking.  It has the following notable features:
 
 - Support for [[file:concepts.org::#ltl][LTL]] (several syntaxes supported) and

doc/org/install.org

@@ -19,8 +19,8 @@ successful development build]].
 
 ** Requirements
 
-Spot requires a C++14-compliant compiler.  =g++= 5.0 or later, as well
+Spot requires a C++17-compliant compiler.  =g++= 7.0 or later, as well
-as =clang++= 3.5 or later should work.
+as =clang++= 5.0 or later should work.
 
 Spot expects a complete installation of Python (version 3.3 or later).
 Especially, Python's headers files should be installed (the package to
@@ -91,7 +91,7 @@ our [[https://www.lrde.epita.fr/repo/debian.gpg][GPG key]].  Its fingerprint is
 9E74 44F2 A84A=, if you want to verify it.
 
 The package =spot= contains the [[file:tools.org][command-line tools]]. =libspot-dev=
-contains the header files if you plan to use Spot in a C++14
+contains the header files if you plan to use Spot in a C++17
 program. =spot-doc= contains some html (including these pages) and pdf
 documentation. Finally =python3-spot= contains some Python bindings
 (this package also installs some ipython notebooks that you can use as

doc/org/tut.org

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 #+TITLE: Code Examples
-#+DESCRIPTION: Directory of code examples for using Spot in C++14, Python, and shell.
+#+DESCRIPTION: Directory of code examples for using Spot in C++17, Python, and shell.
 #+INCLUDE: setup.org
 #+HTML_LINK_UP: index.html
 

doc/org/upgrade2.org

@@ -17,10 +17,10 @@ experience of updating a couple of projects that are using Spot.
 * Overview of the changes
 
   Let's begin by just trying to summarize what has changed between
-  Spot 1.2.6 and Spot 2.0, just to get an idea of what will need to be
+  Spot 1.2.6 and Spot 2.x, just to get an idea of what will need to be
   updated.
 
-  1. [[#cpp14][Spot now compiles using the C++14 standard]].  Compliant compiler
+  1. [[#cpp17][Spot now compiles using the C++17 standard]].  Compliant compilers
      are sufficiently widespread now that this should not be an issue.
 
   2. The layout of the source-tree and the layout of the installed
@@ -83,22 +83,20 @@ experience of updating a couple of projects that are using Spot.
      numbered.  Many algorithms have been rewritten on top of this
      =twa_graph= class, and this simplified them a lot.
 
-* Upgrading to C++14
+* Upgrading to C++17
   :PROPERTIES:
-  :CUSTOM_ID: cpp14
+  :CUSTOM_ID: cpp17
   :END:
 
-  Because Spot now relies on C++14 features, programs that use Spot
+  Because Spot now relies on C++17 features, programs that use Spot
   should at least be compiled using this version (or a later one) of
   the language.
 
-  Before the =g++= 6.0, the default C++ standard used was C++98, and
+  This is usually done by passing the option =-std=c++17= to =g++= or
-  enabling C++14 is usually done by passing the option =-std=c++14=.
+  =clang++=.
-  In =g++= 6.0 the default C++ standard used is C++14, so passing
-  =-std=c++14= is not necessary.
 
-  Upgrading from C++98, C++03 or C++11 to C++14 should be relatively
+  Upgrading from C++98, C++03, C++11, or C++14 to C++17 should be
-  smooth as the language is /mostly/ backward compatible.
+  relatively smooth as the language is /mostly/ backward compatible.
 
 * Upgrading =#include= directives
   :PROPERTIES:

spot/misc/escape.hh

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2011-2013, 2015, 2018 Laboratoire de Recherche et
+// Copyright (C) 2011-2013, 2015, 2018, 2020 Laboratoire de Recherche et
 // Developpement de l'Epita (LRDE).
 // Copyright (C) 2004  Laboratoire d'Informatique de Paris 6 (LIP6),
 // département Systèmes Répartis Coopératifs (SRC), Université Pierre
@@ -36,7 +36,7 @@ namespace spot
   /// In CSV files, as defined by RFC4180, double-quoted string that
   /// contain double-quotes should simply duplicate those quotes.
   ///
-  /// Note that in C++14,
+  /// Note that since C++14,
   /// ```
   /// os << std::quoted(str, '"', '"');
   /// ```