protect new cons cell in named StretchyList::push_back()

The named variant of push_back() allocated the new cons cell before
constructing the Symbol for its tag. If the tag name was not yet
interned, Rf_install() could allocate and trigger a garbage collection
that reclaimed the still-unreachable cell. Construct the Symbol first,
matching push_front().

Fixes #1489.

Author: kevinushey

ChangeLog

@@ -1,3 +1,10 @@
+2026-08-02  Kevin Ushey <kevinushey@gmail.com>
+
+	* inst/include/Rcpp/api/meat/StretchyList.h: Protect the new cell
+	from collection during Rf_install() in named push_back() (#1489)
+	* inst/tinytest/cpp/misc.cpp: Add regression test
+	* inst/tinytest/test_misc.R: Idem
+
 2026-07-24  Dirk Eddelbuettel  <edd@debian.org>
 
 	* DESCRIPTION (Version, Date): Roll micro version and date

inst/include/Rcpp/api/meat/StretchyList.h

@@ -35,8 +35,8 @@ namespace Rcpp{
     template< typename T>
     StretchyList_Impl<StoragePolicy>& StretchyList_Impl<StoragePolicy>::push_back__impl( const T& obj, traits::true_type ){
         Shield<SEXP> s( wrap(obj.object) ) ;
-        SEXP tmp  = Rf_cons( s, R_NilValue );
         Symbol tag  = obj.name ;
+        SEXP tmp  = Rf_cons( s, R_NilValue );
         SET_TAG(tmp, tag) ;
         SEXP self = Storage::get__() ;
         SETCDR( CAR(self), tmp) ;

inst/tinytest/cpp/misc.cpp

@@ -173,6 +173,13 @@ StretchyList named_stretchy_list() {
     return out;
 }
 
+// [[Rcpp::export]]
+StretchyList named_stretchy_list_dynamic(std::string name) {
+    StretchyList out;
+    out.push_back( Named(name, 42) );
+    return out;
+}
+
 // [[Rcpp::export]]
 void test_stop_variadic() {
     stop( "%s %d", "foo", 3 );

inst/tinytest/test_misc.R

@@ -129,6 +129,17 @@ expect_equal(stretchy_list(), pairlist( "foo", 1L, 3.2 ))
 #    test.named_StretchyList <- function(){
 expect_equal(named_stretchy_list(), pairlist( a = "foo", b = 1L, c = 3.2 ))
 
+#    test.named_StretchyList_gc <- function(){
+## push_back() must keep the new cell protected across the Rf_install()
+## needed for a not-yet-interned tag name (#1489)
+name <- paste(sample(c(letters, LETTERS), 32, TRUE), collapse = "")
+gctorture(TRUE)
+result <- named_stretchy_list_dynamic(name)
+gctorture(FALSE)
+expected <- pairlist(42L)
+names(expected) <- name
+expect_equal(result, expected)
+
 #    test.stop.variadic <- function(){
 m <- tryCatch( test_stop_variadic(), error = function(e){
     conditionMessage(e)