Support C++26

Tested locally and had success with R-devel but not R-release but that may
well be due to my R-release being the packaged .deb built with a more
conservative g++ version.  A local R 4.5.0 should also work with proper g++
or clang++

Author: eddelbuettel

ChangeLog

@@ -1,3 +1,7 @@
+2025-05-06  Dirk Eddelbuettel  <[email protected]>
+
+	* R/Attributes.R: Support C++26 via plugin
+
 2025-05-05  Dirk Eddelbuettel  <[email protected]>
 
 	* inst/tinytest/test_sugar.R: Condition four NA-related tests away on

R/Attributes.R

@@ -560,6 +560,13 @@ compileAttributes <- function(pkgdir = ".", verbose = getOption("verbose")) {
         list(env = list(PKG_CXXFLAGS ="-std=c++23"))
 }
 
+# built-in C++26 plugin for C++26
+.plugins[["cpp26"]] <- function() {
+    if (getRversion() >= "4.5")         # with recent R versions, R can decide
+        list(env = list(USE_CXX26 = "yes"))
+    else
+        list(env = list(PKG_CXXFLAGS ="-std=c++26"))
+}
 
 ## built-in C++1z plugin for C++17 standard under development
 ## note that as of Feb 2017 this is taken to be a moving target