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.

Add a long-index overload that is only compiled when
LONG_VECTOR_SUPPORT is not defined, so LP64 builds (where R_xlen_t
== long already) are unaffected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jeroen

ChangeLog

@@ -1,3 +1,10 @@
+2026-06-23  Jeroen Ooms  <jeroen@berkeley.edu>
+
+	* inst/include/Rcpp/vector/Vector.h (Vector::operator[]): Add a
+	long-index overload on platforms without LONG_VECTOR_SUPPORT (such
+	as wasm32) to avoid ambiguity with the built-in pointer subscript
+	synthesised via the implicit Vector -> SEXP conversion
+
 2026-06-18  Dirk Eddelbuettel  <edd@debian.org>
 
 	* vignettes/rmd/Rcpp.bib: Updated references

inst/include/Rcpp/vector/Vector.h

@@ -337,6 +337,14 @@ 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, which makes c[i] with a long index
+    // ambiguous between this operator[] and the built-in pointer subscript
+    // synthesised via the implicit Vector -> SEXP conversion.
+    inline Proxy operator[]( long i ){ return cache.ref(i) ; }
+    inline const_Proxy operator[]( long i ) const { return cache.ref(i) ; }
+#endif
 
     inline Proxy operator()( const size_t& i) {
         return cache.ref( offset(i) ) ;