Fix assume() chneck for clang C++17/C++20 build.

In clang 19, all of a sudden the compile-time feature check for
`[[assume()]]` support started issuing a warning: it says that
attribute is a "C++23 extension."  And the maintainer-mode build treats
warnings as wrrors, so...  Kaboom.

The problem is that in this mode, clang 19 does define the feature test
macro for this attribute even in C++ 17, but when you actually _use_ the
attribute, clang warns (at least with strict checking options) that it
is not part of C++17 (or C++20, as the case may be).

To get around that, I wrote a more substantial feature check and ran it
at _configure_ time.  This is where my work on systematising the feature
check mechanism really pays off — a few years ago this would have been a
fairly substantial change, with lots of opportunities to do it wrong.

cmake/pqxx_cxx_feature_checks.cmake

@@ -1,5 +1,9 @@
 # Configuration for feature checks. Generated by generate_check_config.py.
 include(CheckCXXSourceCompiles)
+try_compile(
+ PQXX_HAVE_ASSUME ${PROJECT_BINARY_DIR}
+ SOURCES ${PROJECT_SOURCE_DIR}/config-tests/PQXX_HAVE_ASSUME.cxx
+)
 try_compile(
  PQXX_HAVE_CHARCONV_FLOAT ${PROJECT_BINARY_DIR}
  SOURCES ${PROJECT_SOURCE_DIR}/config-tests/PQXX_HAVE_CHARCONV_FLOAT.cxx

config-tests/PQXX_HAVE_ASSUME.cxx

@@ -0,0 +1,5 @@
+int main(int argc, char **argv)
+{
+ [[assume(argv != nullptr)]];
+ return argc - 1;
+}

configitems

@@ -4,6 +4,7 @@ PACKAGE_NAME internal autotools
 PACKAGE_STRING internal autotools
 PACKAGE_TARNAME internal autotools
 PACKAGE_VERSION internal autotools
+PQXX_HAVE_ASSUME public compiler
 PQXX_HAVE_CHARCONV_INT internal compiler
 PQXX_HAVE_CHARCONV_FLOAT internal compiler
 PQXX_HAVE_CMP public compiler

configure

@@ -17223,6 +17223,34 @@ fi # End of gcc-specific part.
 
 
 # Configuration for feature checks. Generated by generate_check_config.py.
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking PQXX_HAVE_ASSUME" >&5
+printf %s "checking PQXX_HAVE_ASSUME... " >&6; }
+PQXX_HAVE_ASSUME=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h. */
+int main(int argc, char **argv)
+
+{
+
+ [[assume(argv != nullptr)]];
+
+ return argc - 1;
+
+}
+
+
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"
+then :
+
+printf "%s\n" "#define PQXX_HAVE_ASSUME 1" >>confdefs.h
+
+else $as_nop
+ PQXX_HAVE_ASSUME=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.beam conftest.$ac_ext
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $PQXX_HAVE_ASSUME" >&5
+printf "%s\n" "$PQXX_HAVE_ASSUME" >&6; }
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking PQXX_HAVE_CHARCONV_FLOAT" >&5
 printf %s "checking PQXX_HAVE_CHARCONV_FLOAT... " >&6; }
 PQXX_HAVE_CHARCONV_FLOAT=yes

include/pqxx/config.h.in

@@ -6,7 +6,7 @@
 /* Define to 1 if you have the <inttypes.h> header file. */
 #undef HAVE_INTTYPES_H
 
-/* Define to 1 if you have the `pq' library (-lpq). */
+/* Define to 1 if you have the 'pq' library (-lpq). */
 #undef HAVE_LIBPQ
 
 /* Define to 1 if you have the <stdint.h> header file. */
@@ -57,6 +57,9 @@
 /* Define to the version of this package. */
 #undef PACKAGE_VERSION
 
+/* Define if this feature is available. */
+#undef PQXX_HAVE_ASSUME
+
 /* Define if this feature is available. */
 #undef PQXX_HAVE_CHARCONV_FLOAT
 
@@ -114,7 +117,7 @@
 /* Define if this feature is available. */
 #undef PQXX_HAVE_YEAR_MONTH_DAY
 
-/* Define to 1 if all of the C90 standard headers exist (not just the ones
+/* Define to 1 if all of the C89 standard headers exist (not just the ones
  required in a freestanding environment). This macro is provided for
  backward compatibility; new code need not use it. */
 #undef STDC_HEADERS

include/pqxx/internal/header-pre.hxx

@@ -181,11 +181,8 @@
 
 
 // C++23: Assume support.
-// C++20: Assume __has_cpp_attribute is defined.
-#if !defined(__has_cpp_attribute)
-# define PQXX_ASSUME(condition) while (false)
-#elif !__has_cpp_attribute(assume)
-# define PQXX_ASSUME(condition) while (false)
-#else
+#if defined(PQXX_HAVE_ASSUME)
 # define PQXX_ASSUME(condition) [[assume(condition)]]
+#else
+# define PQXX_ASSUME(condition) while (false)
 #endif

pqxx_cxx_feature_checks.ac

@@ -1,4 +1,14 @@
 # Configuration for feature checks. Generated by generate_check_config.py.
+AC_MSG_CHECKING([PQXX_HAVE_ASSUME])
+PQXX_HAVE_ASSUME=yes
+AC_COMPILE_IFELSE(
+ [read_test(PQXX_HAVE_ASSUME.cxx)],
+ AC_DEFINE(
+ [PQXX_HAVE_ASSUME],
+ 1,
+ [Define if this feature is available.]),
+ PQXX_HAVE_ASSUME=no)
+AC_MSG_RESULT($PQXX_HAVE_ASSUME)
 AC_MSG_CHECKING([PQXX_HAVE_CHARCONV_FLOAT])
 PQXX_HAVE_CHARCONV_FLOAT=yes
 AC_COMPILE_IFELSE(