[BUG]: does not work with Eigen 5

[e-kwsm]

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What version (or hash if on master) of pybind11 are you using?

98e50b7

Problem description

Eigen 5 was released on 30th Sep 2025 https://gitlab.com/libeigen/eigen/-/releases/5.0.0.

The following macros are no longer available:

pybind11/tools/FindEigen3.cmake

Lines 36 to 44 in 98e50b7

	string(REGEX MATCH "define[ \t]+EIGEN_WORLD_VERSION[ \t]+([0-9]+)" _eigen3_world_version_match
	"${_eigen3_version_header}")
	set(EIGEN3_WORLD_VERSION "${CMAKE_MATCH_1}")
	string(REGEX MATCH "define[ \t]+EIGEN_MAJOR_VERSION[ \t]+([0-9]+)" _eigen3_major_version_match
	"${_eigen3_version_header}")
	set(EIGEN3_MAJOR_VERSION "${CMAKE_MATCH_1}")
	string(REGEX MATCH "define[ \t]+EIGEN_MINOR_VERSION[ \t]+([0-9]+)" _eigen3_minor_version_match
	"${_eigen3_version_header}")
	set(EIGEN3_MINOR_VERSION "${CMAKE_MATCH_1}")

namely version detection fails.

Reproducible example code

$ cmake -S . -B build
…
-- Eigen3 version .. found in /usr/include/eigen3, but at least version 3.2.7 is required
-- Building tests WITHOUT Eigen, use -DDOWNLOAD_EIGEN=ON to download
…

Is this a regression? Put the last known working version here if it is.

Not a regression

[rwgk]

I hope @henryiii can help here.

This is what Cursor is saying:

Likely this is a CMake/discovery fix. The issue report points at tools/FindEigen3.cmake, and that file still parses the old EIGEN_WORLD_VERSION / EIGEN_MAJOR_VERSION / EIGEN_MINOR_VERSION macros, which Eigen 5 apparently removed. I don’t see pybind11’s Eigen headers depending on those old macros directly; the C++ side seems to use EIGEN_VERSION_AT_LEAST(...) only. tests/CMakeLists.txt already prefers find_package(Eigen3 ... CONFIG) and only falls back to tools/FindEigen3.cmake, so my guess is: fix or bypass that fallback for Eigen 5, then do one real Eigen 5 configure/build to check for any secondary breakage.

Best would be if someone sends a PR with CI additions or changes, so that we have jobs to build and tests against Eigen 5.