Fix ambiguous Vector::operator[] on platforms without LONG_VECTOR_SUPPORT (wasm32)

[jeroen]

Problem

On the wasm32 target (e.g. r-universe webR builds), packages that subscript an Rcpp::Vector with an integer index whose type is not exactly R_xlen_t now fail to compile with an overload-resolution ambiguity. The originally reported failure was in ulid with a long index:

wrapper.cpp:33:10: error: use of overloaded operator '[]' is ambiguous
   (with operand types 'Rcpp::CharacterVector' (aka 'Vector<16>') and 'long')
   33 |         c[i] = ulid::Marshal(ulid::CreateNowRand());

with candidates:

• Vector::operator[](R_xlen_t) — inst/include/Rcpp/vector/Vector.h:338
• built-in operator[](SEXPREC*, __ptrdiff_t) — synthesised because Rcpp::Vector has an implicit operator SEXP() const via PreserveStorage.

The same shape of error also occurs in Rcpp's own headers (newDateVector.h, newDatetimeVector.h, module/Module.h, module/class.h, …) any time a member uses a size_t loop variable to subscript a Vector.

Root cause

R defines R_xlen_t as ptrdiff_t only when LONG_VECTOR_SUPPORT is set (i.e. sizeof(size_t) > 4). On wasm32 that macro is not defined and R_xlen_t falls back to int, while ptrdiff_t remains long. So for c[i] with i of any integer type wider than int:

• Member candidate: identity on object, integral conversion on the index.
• Built-in candidate: user-defined conversion Vector -> SEXP on object, exact match (or narrower conversion) on the index.

The two implicit conversion sequences are mutually unrankable, so the call is ambiguous. On LP64 platforms R_xlen_t == ptrdiff_t == long, the member matches a long index exactly and wins outright; the ambiguity is wasm-specific.

Fix

Add a SFINAE-constrained template overload of Vector::operator[] that accepts any integral index type other than R_xlen_t itself and delegates to the existing R_xlen_t overload with a static_cast. The template provides an identity match on the index argument, so it beats both:

• the non-template R_xlen_t overload (which would require an integral conversion), and
• the built-in pointer subscript (which would require a user-defined conversion on the object).

When the index is exactly R_xlen_t, the SFINAE constraint excludes the template and the existing non-template overload is selected as before.

The new overload is compiled only when LONG_VECTOR_SUPPORT is not defined, so the overload set on LP64 / LLP64 platforms is unchanged. Marking operator SEXP() explicit would be the alternative root-cause fix but is far too disruptive — implicit SEXP conversion is load-bearing for nearly every downstream package.

Test plan

• Existing CI matrix (Linux x86_64/arm64, macOS x86_64/arm64, Windows) — overload set unchanged on LP64/LLP64.
• r-universe wasm build of an affected package (e.g. ulid) succeeds after this patch.

🤖 Generated with Claude Code

[eddelbuettel]

One gentle nudge for change inline.