restrict.h calls index2sub<D> without including batch.h — g++ rejects the whole header

[balbasty]

Found while standing up the ASan/UBSan CI lane for fastfields-cpu-lib (#75). Not a runtime bug — a portability one, and a cheap fix.

What happens

restrict.h line 68 calls index2sub<D>:

offset_t sub[D]; index2sub<D>(j, isize, sub);

but its include list is only

#include "cuda_switch.h"
#include "spline.h"
#include "bounds.h"

index2sub lives in batch.h. The sibling resize.h, which makes the identical call at its line 78, does include it — with a comment naming exactly this dependency:

#include "utils.h" // prod
#include "batch.h" // index2sub

So restrict.h compiles today only because fastfields-cpu-impl/restrict.h happens to include kernels/batch.h — and, in fact, only under clang: cpu-impl includes kernels/restrict.h before kernels/batch.h, so at the point restrict.h is parsed the name is not yet declared.

Evidence

clang++ -std=c++11 (every CI job today) is happy. g++ 13.3 is not:

$ g++ -std=c++11 -O1 -DFF_STATIC_BOUNDS=0 -DFF_STATIC_SPLINES=0 -DFF_TEST_SPARSE -I. -c restrict.cpp
In file included from impl/restrict.h:4,
 from restrict.cpp:8:
impl/kernels/restrict.h: In static member function 'static void ff::cpu::restrict::Multiscale<D, U, IX, IY, IZ>::restrict(...)':
impl/kernels/restrict.h:68:30: error: 'index2sub' was not declared in this scope
 68 | offset_t sub[D]; index2sub<D>(j, isize, sub);
 | ^~~~~~~~~

gcc's two-phase lookup binds the non-dependent template-name at definition context; clang is more forgiving here. Only restrict.cpp fails — the other eight modules build clean under g++.

Why it matters

It is one line, but it costs a real option: a sanitizer CI lane built with g++ needs no extra packages (libasan/libubsan ship with the compiler), whereas the clang equivalent needs a release-numbered libclang-rt-<N>-dev on top of clang. The lane added in fastfields-cpu-lib#84 went with clang + that extra package for exactly this reason.

Fix

 #include "cuda_switch.h"
 #include "spline.h"
 #include "bounds.h"
+#include "batch.h" // index2sub

mirroring resize.h. Worth a quick audit of the other module headers for the same include-what-you-use gap while in there.

🤖 Generated with Claude Code

https://claude.ai/code/session_016AjQcY78NgbagPSbPJRr6Z