Feature-test macros + config header!

We're getting bug reports (e.g. #732) for situations where people have a
libpqxx compiled as C++17 but an application compiled as C++20, or _vice
versa._

Generally it's probably not wise to link C++17-compiled code to code
compiled as C++20.  But two things I did exacerbate the problem:
1. I moved a bunch of C++ feature checks to compile time, using C++20's
   feature test macros.  It looked like a much cleaner, easier way
   forward with reduced maintenance overhead.
2. Usage of C++20's `std::source_location` _when present_ affects the
   ABI.  So effectively there's one ABI with, and one without.  I see
   that mostly as the price of doing libraries in C++ — it's generally
   dangerous to package library binaries, unless they've been designed
   to be compatible, or they come as a range of binaries for various
   compilers, versions, and configurations.

And the real problem is that _these two changes interacted._  The
detection of support for `std::source_location` happened at compile
time.  And so if you compile libpqxx in C++17 (without this feature)
and the application in C++20 (with this feature), the two will not be
link-compatible!

In this first commit I'm prototyping a new approach that I hope will
combine the ease of maintenance of feature test macros with the ABI
stability of a configuration header.  Configuration speed should lie
somewhere inbetween: no more compiling separate little checks for
every individual C++ feature.

But it's not easy.  There are 2 orthogonal binary choices, leaving me
with 4 scenarios to support:
* Autoconf vs. CMake
* Cross-compiled vs. native.

How does cross compilation factor into it?  It works like this: I need
to check C++ feature test macros.  I check them in C++.  But I don't
want to keep editing that C++ code for every feature — there's a
repetitive preprocessor dance that I don't think I could simplify to one
simple line of code, because I'd need to pass macro names as parameters
to macros.  So, I write a minimal config file and run it through a
Python script that generates the C++ code for me.  Then I have the build
configuration machinery compile _and run_ that C++ code, and generate
the configuration header.

Yes, alright, but how does cross compilation factor into it?  First, if
you're cross-compiling, it's not a given that you can _run_ the binary
you produce on the same system!  The whole point is that the two systems
are different.  And two, you'll have a native compilation environment
but there's no guarantee that it will resemble the cross compilation
environment at all.  So if you compile a binary to run locally, you may
get very different results.

So for cross-compilation, the Python script just generates a minimal
configuration header that just has all features disabled.  And in this
first commit I've got that working for autoconf.  But I'm still
struggling with CMake (thanks @KayEss for helping with this).

If it gets too difficult, I may go a different route: generate C++ code
from Python, but only run it through the preprocessor.  (I believe
autoconf has a standard, portable way of running the preprocessor; let's
hope CMake has one as well.)  The output will still C++ code, but now
it's been preprocessed, so hopefully it'll be possible to tell portably
which features are present.  And ironically, I think I'd then have to
have another Python script to _postprocess_ the preprocessed code and
turn it into a ready-to-use config header.

aclocal.m4

@@ -20,6 +20,65 @@ You have another version of autoconf.  It may work, but is not guaranteed to.
 If you have problems, you may need to regenerate the build system entirely.
 To do so, use the procedure documented by the package, typically 'autoreconf'.])])
 
+# ===========================================================================
+#     https://www.gnu.org/software/autoconf-archive/ax_file_escapes.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_FILE_ESCAPES
+#
+# DESCRIPTION
+#
+#   Writes the specified data to the specified file.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Tom Howard <tomhoward@users.sf.net>
+#
+#   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 8
+
+AC_DEFUN([AX_FILE_ESCAPES],[
+AX_DOLLAR="\$"
+AX_SRB="\\135"
+AX_SLB="\\133"
+AX_BS="\\\\"
+AX_DQ="\""
+])
+
+# ===========================================================================
+#     https://www.gnu.org/software/autoconf-archive/ax_print_to_file.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_PRINT_TO_FILE([FILE],[DATA])
+#
+# DESCRIPTION
+#
+#   Writes the specified data to the specified file.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Tom Howard <tomhoward@users.sf.net>
+#
+#   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 8
+
+AC_DEFUN([AX_PRINT_TO_FILE],[
+AC_REQUIRE([AX_FILE_ESCAPES])
+printf "$2" > "$1"
+])
+
 # Copyright (C) 2002-2021 Free Software Foundation, Inc.
 #
 # This file is free software; the Free Software Foundation

configure

@@ -1933,6 +1933,49 @@ printf "%s\n" "$ac_res" >&6; }
   eval $as_lineno_stack; ${as_lineno_stack:+:} unset as_lineno
 
 } # ac_fn_cxx_check_header_compile
+
+# ac_fn_cxx_try_run LINENO
+# ------------------------
+# Try to run conftest.$ac_ext, and return whether this succeeded. Assumes that
+# executables *can* be run.
+ac_fn_cxx_try_run ()
+{
+  as_lineno=${as_lineno-"$1"} as_lineno_stack=as_lineno_stack=$as_lineno_stack
+  if { { ac_try="$ac_link"
+case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:${as_lineno-$LINENO}: $ac_try_echo\""
+printf "%s\n" "$ac_try_echo"; } >&5
+  (eval "$ac_link") 2>&5
+  ac_status=$?
+  printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; } && { ac_try='./conftest$ac_exeext'
+  { { case "(($ac_try" in
+  *\"* | *\`* | *\\*) ac_try_echo=\$ac_try;;
+  *) ac_try_echo=$ac_try;;
+esac
+eval ac_try_echo="\"\$as_me:${as_lineno-$LINENO}: $ac_try_echo\""
+printf "%s\n" "$ac_try_echo"; } >&5
+  (eval "$ac_try") 2>&5
+  ac_status=$?
+  printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }
+then :
+  ac_retval=0
+else $as_nop
+  printf "%s\n" "$as_me: program exited with status $ac_status" >&5
+       printf "%s\n" "$as_me: failed program was:" >&5
+sed 's/^/| /' conftest.$ac_ext >&5
+
+       ac_retval=$ac_status
+fi
+  rm -rf conftest.dSYM conftest_ipa8_conftest.oo
+  eval $as_lineno_stack; ${as_lineno_stack:+:} unset as_lineno
+  as_fn_set_status $ac_retval
+
+} # ac_fn_cxx_try_run
 ac_configure_args_raw=
 for ac_arg
 do
@@ -3571,7 +3614,6 @@ ac_config_headers="$ac_config_headers include/pqxx/config.h"
 # Read test programme from config-test.
 
 
-
 # Checks for programs.
 
 
@@ -17600,6 +17642,7 @@ printf "%s\n" "$cxa_demangle" >&6; }
 
 
 # C++20: Assume support.
+# XXX: Replace with generated pqxx_have_concepts.
 # Check for sufficient Concepts support, introduced with C++20.
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking concepts" >&5
 printf %s "checking concepts... " >&6; }
@@ -17664,6 +17707,7 @@ printf "%s\n" "$concepts" >&6; }
 
 
 # C++20: Assume support.
+# XXX: Replace with generated pqxx_have_span.
 # Check for C++20 std::span.
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking std::span" >&5
 printf %s "checking std::span... " >&6; }
@@ -18807,6 +18851,118 @@ rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 printf "%s\n" "yes" >&6; }
 
+# Configuration based on C++ feature test macros.  We want a resuable header
+# describing these, so users can build a libpqxx with a fixed, stable config
+# even when actual compile options may differ.
+#
+# Unfortunately this will not work when cross-compiling.  There is just no
+# guarantee that compilation for the machine where we're building will be
+# at all similar to compilation for the target architecture.  In that case,
+# we will generate a dummy config header with all features disabled.
+mkdir -p include/pqxx
+
+AX_DOLLAR="\$"
+AX_SRB="\\135"
+AX_SLB="\\133"
+AX_BS="\\\\"
+AX_DQ="\""
+
+if test "$cross_compiling" = yes
+then :
+  # We're cross-compiling.  Generate a dummy.
+
+
+printf "// Dummy settings for cross-compile.  Configure manually.
+#define pqxx_have_concepts false
+#define pqxx_have_multidim false
+#define pqxx_have_source_location false
+#define pqxx_have_span false
+#define pqxx_have_ssize false
+#define pqxx_have_unreachable false
+" > "include/pqxx/cxx_features.hxx"
+
+
+else $as_nop
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+#include <fstream>
+
+int main()
+{
+    std::ofstream("include/pqxx/cxx_features.hxx") <<
+
+        // Feature test macros for libpqxx.
+        // Generated by generate_checks.py.
+
+"#define pqxx_have_concepts "
+#if defined(__cpp_concepts) && __cpp_concepts
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+"#define pqxx_have_multidim "
+#if defined(__cpp_multidimensional_subscript) && __cpp_multidimensional_subscript
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+"#define pqxx_have_source_location "
+#if defined(__cpp_lib_source_location) && __cpp_lib_source_location
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+"#define pqxx_have_span "
+#if defined(__cpp_lib_span) && __cpp_lib_span
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+"#define pqxx_have_ssize "
+#if defined(__cpp_lib_ssize) && __cpp_lib_ssize
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+"#define pqxx_have_unreachable "
+#if defined(__cpp_lib_unreachable) && __cpp_lib_unreachable
+    "true"
+#else
+    "false"
+#endif
+"\n"
+
+    << std::endl;
+}
+
+
+
+_ACEOF
+if ac_fn_cxx_try_run "$LINENO"
+then :
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: Generated cxx_features.hxx." >&5
+printf "%s\n" "Generated cxx_features.hxx." >&6; }
+else $as_nop
+  { { printf "%s\n" "$as_me:${as_lineno-$LINENO}: error: in \`$ac_pwd':" >&5
+printf "%s\n" "$as_me: error: in \`$ac_pwd':" >&2;}
+as_fn_error $? "\"C++ features check failed.\"
+See \`config.log' for more details" "$LINENO" 5; }
+fi
+rm -f core *.core core.conftest.* gmon.out bb.out conftest$ac_exeext \
+  conftest.$ac_objext conftest.beam conftest.$ac_ext
+fi
+
+
 
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether ${MAKE-make} sets \$(MAKE)" >&5
 printf %s "checking whether ${MAKE-make} sets \$(MAKE)... " >&6; }
@@ -18844,7 +19000,6 @@ fi
 ac_config_files="$ac_config_files Makefile config/Makefile doc/Makefile doc/Doxyfile src/Makefile test/Makefile tools/Makefile include/Makefile include/pqxx/Makefile libpqxx.pc"
 
 
-
 ac_config_commands="$ac_config_commands configitems"
 
 

configure.ac

@@ -24,7 +24,6 @@ AC_PREFIX_DEFAULT(/usr/local)
 AC_DEFUN([read_test], [AC_LANG_SOURCE(
 	esyscmd(tools/m4esc.py --input=config-tests/$1))])
 
-
 # Checks for programs.
 AC_PROG_CXX
 AC_PROG_INSTALL
@@ -300,6 +299,7 @@ AC_MSG_RESULT($cxa_demangle)
 
 
 # C++20: Assume support.
+# XXX: Replace with generated pqxx_have_concepts.
 # Check for sufficient Concepts support, introduced with C++20.
 AC_MSG_CHECKING([concepts])
 concepts=yes
@@ -314,6 +314,7 @@ AC_MSG_RESULT($concepts)
 
 
 # C++20: Assume support.
+# XXX: Replace with generated pqxx_have_span.
 # Check for C++20 std::span.
 AC_MSG_CHECKING([std::span])
 span=yes
@@ -697,6 +698,34 @@ change!
 ])])
 AC_MSG_RESULT(yes)
 
+# Configuration based on C++ feature test macros.  We want a resuable header
+# describing these, so users can build a libpqxx with a fixed, stable config
+# even when actual compile options may differ.
+#
+# Unfortunately this will not work when cross-compiling.  There is just no
+# guarantee that compilation for the machine where we're building will be
+# at all similar to compilation for the target architecture.  In that case,
+# we will generate a dummy config header with all features disabled.
+mkdir -p include/pqxx
+AC_RUN_IFELSE(
+	[AC_LANG_SOURCE(
+		esyscmd(
+			"tools/generate_checks.py" \
+			"cxx_features.txt" \
+			"-H" "include/pqxx/cxx_features.hxx"
+		)
+	)],
+	[AC_MSG_RESULT([Generated cxx_features.hxx.])],
+	[AC_MSG_FAILURE(["C++ features check failed."])],
+	# We're cross-compiling.  Generate a dummy.
+	[AX_PRINT_TO_FILE(
+		include/pqxx/cxx_features.hxx,
+		esyscmd(
+			"tools/generate_checks.py" \
+			"cxx_features.txt" \
+			"--dummy"))]
+)
+
 
 AC_PROG_MAKE_SET
 
@@ -705,7 +734,6 @@ AC_CONFIG_FILES([
 	test/Makefile tools/Makefile include/Makefile include/pqxx/Makefile
 	libpqxx.pc])
 
-
 AC_CONFIG_COMMANDS([configitems], ["${srcdir}/tools/splitconfig" "${srcdir}"])
 
 AC_CONFIG_FILES([compile_flags])

cxx_features.txt

@@ -0,0 +1,6 @@
+__cpp_concepts pqxx_have_concepts
+__cpp_multidimensional_subscript pqxx_have_multidim
+__cpp_lib_source_location pqxx_have_source_location
+__cpp_lib_span pqxx_have_span
+__cpp_lib_ssize pqxx_have_ssize
+__cpp_lib_unreachable pqxx_have_unreachable