Use __cpp_has_attribute() (#928). (#929)

Rewrite the feature checks for `[[assume]]`, `[[likely]]`, and `[[unlikely]]` using the C++20 `__cpp_has_attribute()` macro.

Unfortunately some compilers will still _warn_ about the attribute ("it's a C++20 extension!") so the checks also need to _try_ the attribute.

NEWS

@@ -2,6 +2,8 @@
  - Fix string conversion buffer budget for arrays containing nulls. (#921)
  - Remove `-fanalyzer` option again; gcc is still broken.
  - Oops, no, minimum CMake version is not 3.28, but 3.12!
+ - Fix buffer overrun in converting array with nulls to string. (#922)
+ - Fix warnings on compilers that accept `[[assume]]` with a warning. (#928)
 7.10.0
  - Deprecate `errorhandler`; replace with lambda-friendly "notice handlers."
  - Deprecate `notification_receiver`; replace with "notification handlers"

config-tests/PQXX_HAVE_ASSUME.cxx

@@ -1,5 +1,12 @@
-int main(int argc, char **argv)
+#if !__has_cpp_attribute(assume)
+#  error "No support for [[assume]] attribute."
+#endif
+
+#include <iostream>
+
+
+void foo(int i)
 {
-  [[assume(argv != nullptr)]];
-  return argc - 1;
+  [[assume(i > 0)]];
+  std::cout << i << '\n';
 }

config-tests/PQXX_HAVE_LIKELY.cxx

@@ -1,16 +1,14 @@
 // Test for C++20 [[likely]] and [[unlikely]] attributes.
 // C++20: Assume support.
 
-int main(int argc, char **)
-{
-#if __cplusplus < 202002L
-  deliberately_fail(because, older, C++, standard);
+#if !__has_cpp_attribute(likely)
+#  error "No support for [[likely]] / [[unlikely]] attributes."
 #endif
 
-  int x = 0;
-  if (argc == 1) [[likely]]
-    x = 0;
+int foo(int i)
+{
+  if (i > 0) [[likely]]
+    return 100;
   else
-    x = 1;
-  return x;
+    return 0;
 }

configure

@@ -17237,13 +17237,27 @@ 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)
+#if !__has_cpp_attribute(assume)
+
+#  error "No support for [[assume]] attribute."
+
+#endif
+
+
+
+#include <iostream>
+
+
+
+
+
+void foo(int i)
 
 {
 
-  [[assume(argv != nullptr)]];
+  [[assume(i > 0)]];
 
-  return argc - 1;
+  std::cout << i << '\n';
 
 }
 
@@ -17583,29 +17597,21 @@ cat confdefs.h - <<_ACEOF >conftest.$ac_ext
 
 
 
-int main(int argc, char **)
-
-{
-
-#if __cplusplus < 202002L
+#if !__has_cpp_attribute(likely)
 
-  deliberately_fail(because, older, C++, standard);
+#  error "No support for [[likely]] / [[unlikely]] attributes."
 
 #endif
 
 
 
-  int x = 0;
-
-  if (argc == 1) [[likely]]
-
-    x = 0;
+int foo(int i)
 
-  else
+{
 
-    x = 1;
+  if (i > 0) [[likely]] return 100;
 
-  return x;
+  else return 0;
 
 }