TASK-001: Bump C++ standard floor to C++20

Raises the project's C++ standard floor from C++17 to C++20 so that
subsequent v2.0 work can rely on concepts, std::span, <bit>,
designated initializers, and std::pmr without per-feature gates.

- m4/ax_cxx_compile_stdcxx.m4: replaced with upstream serial 25
  (autoconf-archive). The vendored serial 12 only accepted [11], [14],
  [17] and m4_fatals on anything else; serial 25 adds [20] and [23]
  alternatives plus the C++20 feature-test bodies.
- configure.ac:47: AX_CXX_COMPILE_STDCXX([17]) -> ([20], [noext],
  [mandatory]). [noext] keeps -std=c++20 (no gnu++20 extensions in
  ABI surface); [mandatory] aborts cleanly on too-old toolchains.
- configure.ac:224: dropped redundant -std=c++17 from the
  --enable-debug AM_CXXFLAGS branch. AX_CXX_COMPILE_STDCXX already
  appends -std=c++20 to $CXX, so leaving the override in would
  silently downgrade debug builds.
- Verified Makefile.am, src/Makefile.am, test/Makefile.am, and
  examples/Makefile.am: no per-subdirectory -std= overrides exist.
- .github/workflows/verify-build.yml:
  - Pruned gcc-9, clang-11, clang-12 matrix rows (incomplete C++20
    support: missing concepts/<bit>/<span> in libstdc++/libc++).
  - Bumped IWYU CXXFLAGS from -std=c++11 to -std=c++20.
- README.md: bumped Requirements to "g++ >= 10 or clang >= 13
  (Apple Clang from Xcode 15+)" and "C++20 or newer". Added a
  one-liner about gcc-toolset-14 on RHEL 9.
- README.CentOS-7: updated to reflect the C++20 floor and the
  gcc-toolset-14 workaround.
- ChangeLog: noted the standard bump under 0.20.0.

Verification (Apple Clang 21 on macOS):
- ./configure && make: succeeds with -std=c++20.
- make check: 17/17 tests pass.
- ./configure --enable-debug && make: clean under
  -Wall -Wextra -Werror -pedantic -std=c++20.
- make check (debug): 17/17 tests pass.
- grep -RE '-std=(c\+\+11|c\+\+14|c\+\+17|gnu\+\+(11|14|17))'
    configure.ac Makefile.am src test  -> zero matches.

Refs: PRD §2 NFR (modern C++ idioms), DR-001.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

.github/workflows/verify-build.yml

@@ -108,16 +108,7 @@ jobs:
             debug: debug
             coverage: nocoverage
             shell: bash
-          - test-group: extra
-            os: ubuntu-latest
-            os-type: ubuntu
-            build-type: none
-            compiler-family: gcc
-            c-compiler: gcc-9
-            cc-compiler: g++-9
-            debug: nodebug
-            coverage: nocoverage
-            shell: bash
+          # gcc-9 dropped: lacks full C++20 support (no concepts library, no std::span, no <bit> features).
           - test-group: extra
             os: ubuntu-latest
             os-type: ubuntu
@@ -168,26 +159,7 @@ jobs:
             debug: nodebug
             coverage: nocoverage
             shell: bash
-          - test-group: extra
-            os: ubuntu-22.04
-            os-type: ubuntu
-            build-type: none
-            compiler-family: clang
-            c-compiler: clang-11
-            cc-compiler: clang++-11
-            debug: nodebug
-            coverage: nocoverage
-            shell: bash
-          - test-group: extra
-            os: ubuntu-22.04
-            os-type: ubuntu
-            build-type: none
-            compiler-family: clang
-            c-compiler: clang-12
-            cc-compiler: clang++-12
-            debug: nodebug
-            coverage: nocoverage
-            shell: bash
+          # clang-11 and clang-12 dropped: incomplete C++20 support (concepts/<bit>/<span> gaps).
           - test-group: extra
             os: ubuntu-22.04
             os-type: ubuntu
@@ -662,7 +634,7 @@ jobs:
         # IWYU always return an error code. If it returns "2" it indicates a success so we manage this within the function below.
         function safe_make_iwyu() {
           {
-            make -k CXX='/usr/local/bin/include-what-you-use -Xiwyu --mapping_file=${top_builddir}/../custom_iwyu.imp' CXXFLAGS="-std=c++11 -DHTTPSERVER_COMPILATION -D_REENTRANT $CXXFLAGS" ;
+            make -k CXX='/usr/local/bin/include-what-you-use -Xiwyu --mapping_file=${top_builddir}/../custom_iwyu.imp' CXXFLAGS="-std=c++20 -DHTTPSERVER_COMPILATION -D_REENTRANT $CXXFLAGS" ;
           } || {
             if [ $? -ne 2 ]; then
               return 1;

ChangeLog

@@ -1,5 +1,10 @@
 Version 0.20.0
 
+	Raised minimum C++ standard to C++20. Build now requires gcc >= 10
+		or clang >= 13 (Apple Clang from Xcode 15+). Updated
+		AX_CXX_COMPILE_STDCXX macro (m4/ax_cxx_compile_stdcxx.m4) to
+		serial 25 to support C++20 detection. Pruned CI matrix rows
+		(gcc-9, clang-11, clang-12) that lack full C++20 support.
 	Raised minimum libmicrohttpd requirement to 1.0.0.
 	Migrated Basic Auth to v3 API (MHD_basic_auth_get_username_password3,
 		MHD_queue_basic_auth_required_response3) with UTF-8 support.

README.CentOS-7

@@ -1,7 +1,8 @@
 ## Cent OS 7 / RHEL 7
 
-CentOS 7 has a lower version of gcc (4.8.7) that is barely C++11 capable and this library
-needs a better compiler.  We recommend at least gcc 5+
+CentOS 7's stock gcc (4.8.7) is far too old: this library requires a C++20 compiler
+(gcc >= 10 or clang >= 13).
 
-We recommend installing devtoolset-8
-https://www.softwarecollections.org/en/scls/rhscl/devtoolset-8/
+Install gcc-toolset-14 (or newer) from the RHEL/CentOS Software Collections and
+`source /opt/rh/gcc-toolset-14/enable` before configuring. The same workaround applies
+to RHEL 9 systems whose stock gcc-11 lacks some C++20 library features.

README.md

@@ -87,12 +87,14 @@ Additionally, clients can specify resource limits on the overall number of conne
 libhttpserver can be used without any dependencies aside from libmicrohttpd.
 
 The minimum versions required are:
-* g++ >= 5.5.0 or clang-3.6
-* C++17 or newer
+* g++ >= 10 or clang >= 13 (Apple Clang from Xcode 15+)
+* C++20 or newer
 * libmicrohttpd >= 1.0.0
 * [Optionally]: for TLS (HTTPS) support, you'll need [libgnutls](http://www.gnutls.org/).
 * [Optionally]: to compile the code-reference, you'll need [doxygen](http://www.doxygen.nl/).
 
+On RHEL 9 (and derivatives), the stock GCC 11 is too old for some C++20 library features the build relies on; install the `gcc-toolset-14` package and `source /opt/rh/gcc-toolset-14/enable` before configuring.
+
 Additionally, for MinGW on windows you will need:
 * libwinpthread (For MinGW-w64, if you use thread model posix then you have this)
 

configure.ac

@@ -44,7 +44,7 @@ AC_LANG([C++])
 AC_SYS_LARGEFILE
 
 # Minimal feature-set required
-AX_CXX_COMPILE_STDCXX([17])
+AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
 
 native_srcdir=$srcdir
 
@@ -221,7 +221,7 @@ AM_LDFLAGS="-lstdc++"
 
 if test x"$debugit" = x"yes"; then
     AC_DEFINE([DEBUG],[],[Debug Mode])
-    AM_CXXFLAGS="$AM_CXXFLAGS -DDEBUG -g -Wall -Wextra -Werror -pedantic -std=c++17 -Wno-unused-command-line-argument -O0"
+    AM_CXXFLAGS="$AM_CXXFLAGS -DDEBUG -g -Wall -Wextra -Werror -pedantic -Wno-unused-command-line-argument -O0"
     AM_CFLAGS="$AM_CXXFLAGS -DDEBUG -g -Wall -Wextra -Werror -pedantic -Wno-unused-command-line-argument -O0"
 else
     AC_DEFINE([NDEBUG],[],[No-debug Mode])

m4/ax_cxx_compile_stdcxx.m4

@@ -10,8 +10,8 @@
 #
 #   Check for baseline language coverage in the compiler for the specified
 #   version of the C++ standard.  If necessary, add switches to CXX and
-#   CXXCPP to enable support.  VERSION may be '11' (for the C++11 standard)
-#   or '14' (for the C++14 standard).
+#   CXXCPP to enable support.  VERSION may be '11', '14', '17', '20', or
+#   '23' for the respective C++ standard version.
 #
 #   The second argument, if specified, indicates whether you insist on an
 #   extended mode (e.g. -std=gnu++11) or a strict conformance mode (e.g.
@@ -36,13 +36,15 @@
 #   Copyright (c) 2016, 2018 Krzesimir Nowak <qdlacz@gmail.com>
 #   Copyright (c) 2019 Enji Cooper <yaneurabeya@gmail.com>
 #   Copyright (c) 2020 Jason Merrill <jason@redhat.com>
+#   Copyright (c) 2021, 2024 Jörn Heusipp <osmanx@problemloesungsmaschine.de>
+#   Copyright (c) 2015, 2022, 2023, 2024 Olly Betts
 #
 #   Copying and distribution of this file, with or without modification, are
 #   permitted in any medium without royalty provided the copyright notice
 #   and this notice are preserved.  This file is offered as-is, without any
 #   warranty.
 
-#serial 12
+#serial 25
 
 dnl  This macro is based on the code from the AX_CXX_COMPILE_STDCXX_11 macro
 dnl  (serial version number 13).
@@ -51,6 +53,8 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
   m4_if([$1], [11], [ax_cxx_compile_alternatives="11 0x"],
         [$1], [14], [ax_cxx_compile_alternatives="14 1y"],
         [$1], [17], [ax_cxx_compile_alternatives="17 1z"],
+        [$1], [20], [ax_cxx_compile_alternatives="20"],
+        [$1], [23], [ax_cxx_compile_alternatives="23"],
         [m4_fatal([invalid first argument `$1' to AX_CXX_COMPILE_STDCXX])])dnl
   m4_if([$2], [], [],
         [$2], [ext], [],
@@ -102,9 +106,18 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
     dnl HP's aCC needs +std=c++11 according to:
     dnl http://h21007.www2.hp.com/portal/download/files/unprot/aCxx/PDF_Release_Notes/769149-001.pdf
     dnl Cray's crayCC needs "-h std=c++11"
+    dnl MSVC needs -std:c++NN for C++17 and later (default is C++14)
     for alternative in ${ax_cxx_compile_alternatives}; do
-      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}"; do
-        cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+      for switch in -std=c++${alternative} +std=c++${alternative} "-h std=c++${alternative}" MSVC; do
+        if test x"$switch" = xMSVC; then
+          dnl AS_TR_SH maps both `:` and `=` to `_` so -std:c++17 would collide
+          dnl with -std=c++17.  We suffix the cache variable name with _MSVC to
+          dnl avoid this.
+          switch=-std:c++${alternative}
+          cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_${switch}_MSVC])
+        else
+          cachevar=AS_TR_SH([ax_cv_cxx_compile_cxx$1_$switch])
+        fi
         AC_CACHE_CHECK(whether $CXX supports C++$1 features with $switch,
                        $cachevar,
           [ac_save_CXX="$CXX"
@@ -148,23 +161,44 @@ AC_DEFUN([AX_CXX_COMPILE_STDCXX], [dnl
 dnl  Test body for checking C++11 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_11],
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+  [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11]
 )
 
-
 dnl  Test body for checking C++14 support
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_14],
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+  [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14]
 )
 
+dnl  Test body for checking C++17 support
+
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_17],
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_11
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
-  _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+  [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17]
+)
+
+dnl  Test body for checking C++20 support
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_20],
+  [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_20]
 )
 
+dnl  Test body for checking C++23 support
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_23],
+  [_AX_CXX_COMPILE_STDCXX_testbody_new_in_11
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_14
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_17
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_20
+   _AX_CXX_COMPILE_STDCXX_testbody_new_in_23]
+)
+
+
 dnl  Tests for new features in C++11
 
 m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
@@ -176,7 +210,21 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_11], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201103L
+// MSVC always sets __cplusplus to 199711L in older versions; newer versions
+// only set it correctly if /Zc:__cplusplus is specified as well as a
+// /std:c++NN switch:
+//
+// https://devblogs.microsoft.com/cppblog/msvc-now-correctly-reports-__cplusplus/
+//
+// The value __cplusplus ought to have is available in _MSVC_LANG since
+// Visual Studio 2015 Update 3:
+//
+// https://learn.microsoft.com/en-us/cpp/preprocessor/predefined-macros
+//
+// This was also the first MSVC version to support C++14 so we can't use the
+// value of either __cplusplus or _MSVC_LANG to quickly rule out MSVC having
+// C++11 or C++14 support, but we can check _MSVC_LANG for C++17 and later.
+#elif __cplusplus < 201103L && !defined _MSC_VER
 
 #error "This is not a C++11 compiler"
 
@@ -467,7 +515,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201402L
+#elif __cplusplus < 201402L && !defined _MSC_VER
 
 #error "This is not a C++14 compiler"
 
@@ -591,7 +639,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_17], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201703L
+#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 201703L
 
 #error "This is not a C++17 compiler"
 
@@ -957,8 +1005,66 @@ namespace cxx17
 
 }  // namespace cxx17
 
-#endif  // __cplusplus < 201703L
+#endif  // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 201703L
+
+]])
+
+
+dnl  Tests for new features in C++20
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_20], [[
+
+#ifndef __cplusplus
+
+#error "This is not a C++ compiler"
+
+#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202002L
+
+#error "This is not a C++20 compiler"
+
+#else
+
+#include <version>
+
+namespace cxx20
+{
+
+// As C++20 supports feature test macros in the standard, there is no
+// immediate need to actually test for feature availability on the
+// Autoconf side.
+
+}  // namespace cxx20
+
+#endif  // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202002L
 
 ]])
 
 
+dnl  Tests for new features in C++23
+
+m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_23], [[
+
+#ifndef __cplusplus
+
+#error "This is not a C++ compiler"
+
+#elif (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
+
+#error "This is not a C++23 compiler"
+
+#else
+
+#include <version>
+
+namespace cxx23
+{
+
+// As C++23 supports feature test macros in the standard, there is no
+// immediate need to actually test for feature availability on the
+// Autoconf side.
+
+}  // namespace cxx23
+
+#endif  // (defined _MSVC_LANG ? _MSVC_LANG : __cplusplus) < 202302L
+
+]])