fix: declare R_getRegisteredNamespace() so the C23 sanitizer build compiles

[krlmlr]

LLM disclosure: this PR was prepared with Claude Code (Claude Opus 4.8).

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

What

Fix the R_getRegisteredNamespace compile error in the Sanitizer job of build-and-check.yml without changing the function used — the original call is correct.

Root cause (verified against the logs)

The R >= 4.6.0 fast path in src/rinterface_extra.c looks up the igraph namespace with R_getRegisteredNamespace("igraph") at three sites. That function is correct, public R API: Writing R Extensions documents it (@eapifun, "R 4.6.0 adds R_getRegisteredNamespace … find and return the namespace environment registered for a name") as the supported replacement for the non-API R_NamespaceRegistry, its signature is const char *, and it is a side-effect-free registry lookup (unlike R_FindNamespace(), which evaluates getNamespace() and loads the namespace).

The Sanitizer job builds against a CSAN R-devel image whose headers report R >= 4.6.0 (so the fast path is compiled) but predate the declaration being added to <Rinternals.h>. Proof from the same build log: R_getVar() — from the same header, added in R 4.5.0 — compiles, while R_getRegisteredNamespace() — added in R 4.6.0 — does not. Under clang -std=gnu2x (C23) an implicit declaration is a hard error:

rinterface_extra.c:2520:9: error: use of undeclared identifier 'R_getRegisteredNamespace'

libR does export the symbol — the non-C23 R-devel jobs (e.g. rcc: ubuntu-26.04 (devel)) compile and link the same call, only warning — so this is purely a missing header declaration on that one snapshot.

Fix

<Rinternals.h> is already included unconditionally via src/rinterface.h, and it is the only header that ever declares this entry point — so adding an #include cannot help a snapshot whose header lacks the line. Instead, declare the prototype locally in the R >= 4.6.0 branch:

extern SEXP R_getRegisteredNamespace(const char *name);
rho = R_getRegisteredNamespace("igraph");

It matches R's signature exactly, so it is a compatible redeclaration wherever the installed header already provides it, and it supplies the missing declaration where it does not. The R < 4.6.0 getNamespace() fallback is unchanged. Net change vs main: the prototype line (plus comments) at the three sites.

Verification

Reproduced locally with R 4.5.3 headers (which, like the CSAN snapshot, do not declare the function):

• Without the prototype: error: implicit declaration of function 'R_getRegisteredNamespace' under gcc -std=gnu2x -Werror=implicit-function-declaration — the same failure as CI.
• With the prototype: compiles clean (-Wall -Wextra -std=gnu2x).

The Sanitizer workflow is also dispatched against this branch to confirm end-to-end on the real CSAN image.

Not addressed here (separate, not this compile bug)

• rcc: ubuntu-26.04 (4.2) / (4.3) — pak resolution fails on removed Bioconductor releases (Enhances: graph) 404ing on the Posit mirror. Environmental.
• rcc: * (devel) check failures — those jobs compile fine (examples run); they fail later in R CMD check on an R-devel check condition unrelated to this compile error.

🤖 Generated with Claude Code