[#2311] detect C++20 in autoconf

And enforce -std=c++20 when building
--with-libyang-cpp or --with-sysrepo-cpp.

1 files changed, 33 insertions, 0 deletions

diff --git a/m4macros/ax_cpp20.m4 b/m4macros/ax_cpp20.m4
new file mode 100644
index 0000000000..da116c9dd4
--- /dev/null
+++ b/m4macros/ax_cpp20.m4
@@ -0,0 +1,33 @@
+AC_DEFUN([AX_ISC_CPP20], [
+    AC_MSG_CHECKING(c++20 support)
+
+    # Save flags.
+    CPPFLAGS_SAVED="${CPPFLAGS}"
+    LIBS_SAVED="${LIBS}"
+
+    # Provide -std=c++20 flag temporarily.
+    CPPFLAGS="${CPPFLAGS} -std=c++20"
+
+    # Simulate the definition of std::derived_from.
+    AC_COMPILE_IFELSE(
+      [AC_LANG_PROGRAM(
+        [#include <concepts>
+         template<class Derived, class Base>
+           concept derived_from =
+             std::is_base_of_v<Base, Derived> &&
+             std::is_convertible_v<const volatile Derived*, const volatile Base*>;
+
+          struct BaseClass {};
+          struct DerivedClass : BaseClass {};
+        ],
+        [static_assert(std::derived_from<BaseClass, DerivedClass> == false);
+         static_assert(std::derived_from<DerivedClass, BaseClass> == true);]
+      )],
+      [AC_MSG_RESULT([yes])
+       CPP20_SUPPORTED=true],
+      [AC_MSG_RESULT([no])
+       CPP20_SUPPORTED=false])
+
+    # Restore flags.
+    CPPFLAGS="${CPPFLAGS_SAVED}"
+])