Fixes #115

configure

@@ -27,8 +27,44 @@ case "$CC" in
     ;;
 esac
 
+# By default CXX_STD is CXX11
+CXX_STD=CXX11
+
+# But if it works, use CXX17
+CXX=`"${R_HOME}"/bin/R CMD config CXX17`
+CXX17STD=`"${R_HOME}"/bin/R CMD config CXX17STD`
+CXX17FLAGS=`"${R_HOME}"/bin/R CMD config CXX17FLAGS`
+if [ "$CXX" != "" -a "$CXX17FLAGS" != "" -a "$CXX17STD" != "" ]; then
+  ptimespec='#include <cstdio>
+#include <ctime>
+// Example adapted from https://en.cppreference.com/w/cpp/chrono/c/timespec_get
+
+namespace std {
+
+        int main()
+        {
+                timespec ts;
+                timespec_get(&ts, TIME_UTC);
+                char buf[100];
+                strftime(buf, sizeof buf, "%D %T", gmtime(&ts.tv_sec));
+                printf("Current time: %s.%09ld UTC", buf, ts.tv_nsec);
+                return 0;
+        }
+
+}'
+  echo "$ptimespec" | ${CXX} -x c++ -c $CXX17FLAGS $CXX17STD - -o /dev/null > /dev/null 2>&1
+  if [ $? -eq 0 ]; then
+    # See https://github.com/r-lib/later/issues/115
+    echo "Using c++17."
+    CXX_STD=CXX17
+  else
+    echo "Cannot use c++17 using default configuration for c++11"
+  fi
+fi
+
+
 # Write to Makevars
-sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|" -e "s|@pkg_cppflags@|$PKG_CPPFLAGS|" \
+sed -e "s|@extra_pkg_libs@|$EXTRA_PKG_LIBS|" -e "s|@pkg_cppflags@|$PKG_CPPFLAGS|" -e "s|@cxx_std@|$CXX_STD|"\
     src/Makevars.in > src/Makevars
 
 # Success

src/Makevars.in

@@ -1,5 +1,4 @@
-# Use C++11 if available
-CXX_STD=CXX11
+CXX_STD=@cxx_std@
 
 PKG_CPPFLAGS = -pthread -DSTRICT_R_HEADERS @pkg_cppflags@
 PKG_LIBS = -pthread @extra_pkg_libs@