Fix ambiguous Vector::operator[] on platforms without LONG_VECTOR_SUPPORT

On wasm32 (and other platforms where LONG_VECTOR_SUPPORT is not
defined) R_xlen_t is int while ptrdiff_t is long. Subscripting an
Rcpp::Vector with a long index then becomes ambiguous between the
member operator[] (which needs a long -> int conversion on the index)
and the built-in pointer subscript operator[](SEXPREC*, ptrdiff_t)
synthesised via the implicit Vector -> SEXP conversion (which needs a
user-defined conversion on the object but matches the index exactly).
The two implicit conversion sequences are mutually unrankable so
overload resolution fails.

Author: jeroen

ChangeLog

@@ -1,3 +1,8 @@
+2026-06-23  Jeroen Ooms  <jeroenooms@gmail.com>
+
+	* Add templated integer-index overload on platforms without
+	LONG_VECTOR_SUPPORT (such as wasm32).
+
 2026-06-18  Dirk Eddelbuettel  <edd@debian.org>
 
 	* vignettes/rmd/Rcpp.bib: Updated references

inst/include/Rcpp/vector/Vector.h

@@ -21,6 +21,7 @@
 #define Rcpp__vector__Vector_h
 
 #include <Rcpp/vector/Subsetter.h>
+#include <type_traits>
 
 namespace Rcpp{
 
@@ -337,6 +338,20 @@ class Vector :
 
     inline Proxy operator[]( R_xlen_t i ){ return cache.ref(i) ; }
     inline const_Proxy operator[]( R_xlen_t i ) const { return cache.ref(i) ; }
+#ifndef LONG_VECTOR_SUPPORT
+    // On platforms without long vector support (notably wasm32) R_xlen_t is
+    // int while ptrdiff_t is long, so subscripting with any wider integer
+    // type would otherwise be ambiguous with the built-in pointer subscript
+    // synthesised via the implicit Vector -> SEXP conversion. The template
+    // overloads below provide an exact-match member candidate for every
+    // integer index type other than R_xlen_t itself.
+    template <typename T>
+    inline typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, R_xlen_t>::value, Proxy>::type
+    operator[]( T i ){ return cache.ref(static_cast<R_xlen_t>(i)) ; }
+    template <typename T>
+    inline typename std::enable_if<std::is_integral<T>::value && !std::is_same<T, R_xlen_t>::value, const_Proxy>::type
+    operator[]( T i ) const { return cache.ref(static_cast<R_xlen_t>(i)) ; }
+#endif
 
     inline Proxy operator()( const size_t& i) {
         return cache.ref( offset(i) ) ;