From 67f366055e92df9f6c9d392540683166218925a9 Mon Sep 17 00:00:00 2001
From: Scott White <whitesscott@comcast.net>
Date: Wed, 1 Jul 2026 19:07:02 -0700
Subject: [PATCH] Strip /usr/include from apt pybind11's interface include dirs

The Debian/Ubuntu `pybind11-dev` package ships a CMake config in which
`pybind11::pybind11_headers` has

    INTERFACE_INCLUDE_DIRECTORIES = ${_IMPORT_PREFIX}/include

and, for the apt install, `_IMPORT_PREFIX` resolves to `/usr`, so
`/usr/include` is propagated to every target that consumes pybind11 via
`pybind11_add_module` (in this repo, `_edgellm_runtime`).

nvcc translates that into `-isystem /usr/include` on the host g++ command
line for each CUDA source in the target.  g++ then reorders its system
include chain, placing `/usr/include` before the libstdc++ header directory
`/usr/include/c++/<ver>`.  libstdc++'s `<cmath>` relies on
`#include_next <math.h>` to reach the C library header; with `/usr/include`
moved to the front, nothing after `<cmath>` in the search order still
contains `math.h`, and the build fails during device_link_stub.cu.o with:

    /usr/include/c++/13/cmath: fatal error: math.h: No such file or directory
       #include_next <math.h>

Reproduced on Ubuntu 24.04 aarch64 (Jetson Thor) with `pybind11-dev`
2.11.1 and CUDA 13.2 + gcc 13.3.  Pip-installed pybind11 is unaffected
because its `_IMPORT_PREFIX` points at the venv's site-packages, not `/usr`.

Fix: after `find_package(pybind11 CONFIG REQUIRED)`, strip `/usr/include`
from the `pybind11::pybind11_headers` target's INTERFACE_INCLUDE_DIRECTORIES.
The real pybind11 headers live in a subdirectory of `_IMPORT_PREFIX/include`
(specifically `pybind11/`) and are still exposed via the include-path
resolution rules; only the bogus root include is removed.  No-op for pip
installs of pybind11.

Verified by:
 - `rm -rf build && cmake ..` (no -Dpybind11_DIR override, so apt pybind11
   at /usr/lib/cmake/pybind11 is picked): configure succeeds and the
   generated experimental/pybind/CMakeFiles/_edgellm_runtime.dir/includes_CUDA.rsp
   no longer contains a bare `-isystem /usr/include`.
 - `make -j _edgellm_runtime`: device_link_stub.cu.o now compiles and the
   `_edgellm_runtime.cpython-312-aarch64-linux-gnu.so` links successfully.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 experimental/pybind/CMakeLists.txt | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/experimental/pybind/CMakeLists.txt b/experimental/pybind/CMakeLists.txt
index 855f8bce..daa94de3 100644
--- a/experimental/pybind/CMakeLists.txt
+++ b/experimental/pybind/CMakeLists.txt
@@ -111,6 +111,31 @@ find_package(
   REQUIRED)
 find_package(pybind11 CONFIG REQUIRED)
 
+# ── Work around Debian/Ubuntu pybind11-dev exporting /usr/include ─────────
+# The apt-installed `pybind11-dev` package ships a CMake config whose
+# `pybind11::pybind11_headers` target sets
+#     INTERFACE_INCLUDE_DIRECTORIES = ${_IMPORT_PREFIX}/include
+# and `_IMPORT_PREFIX` resolves to `/usr`, so `/usr/include` gets propagated
+# to every consumer via `-isystem`. When nvcc then invokes host g++ on a CUDA
+# source, `-isystem /usr/include` reorders g++'s system header chain and
+# places `/usr/include` before `/usr/include/c++/<ver>`. libstdc++'s <cmath>
+# uses `#include_next <math.h>`, which now finds nothing after itself in the
+# search order, and compilation fails with:
+#   /usr/include/c++/13/cmath: fatal error: math.h: No such file or directory
+# Filter `/usr/include` out of the interface so the real pybind11 headers
+# (in a subdirectory under _IMPORT_PREFIX) are still exposed but the bogus
+# root include is not. No-op for pip-installed pybind11 (its _IMPORT_PREFIX
+# points at the venv's site-packages, not /usr).
+if(TARGET pybind11::pybind11_headers)
+  get_target_property(_pybind11_iface_inc pybind11::pybind11_headers
+                      INTERFACE_INCLUDE_DIRECTORIES)
+  if(_pybind11_iface_inc)
+    list(REMOVE_ITEM _pybind11_iface_inc "/usr/include")
+    set_property(TARGET pybind11::pybind11_headers PROPERTY
+                 INTERFACE_INCLUDE_DIRECTORIES "${_pybind11_iface_inc}")
+  endif()
+endif()
+
 # ── Header include paths (from source tree) ───────────────────────────────
 set(EDGELLM_INCLUDE_DIRS
     ${EDGELLM_ROOT}/cpp ${EDGELLM_ROOT}/examples/multimodal