build: support Eigen 5 (#6036)

* build: support Eigen 5

fix #6034

* build: probe Eigen 3 and 5 separately in CMake config mode

Avoid relying on package-specific handling of a bounded version range when discovering Eigen through Eigen3Config.cmake.

Made-with: Cursor

* [skip ci] build: clarify Eigen 5 module fallback comment

Explain that the MODULE-mode fallback only exists for older Eigen 3 setups so the remaining fallback path does not look like an unresolved Eigen 5 issue.

Made-with: Cursor

* [skip ci] docs: add Eigen 5 entry to v3.0.4 changelog

Document the Eigen 5 CMake package detection fix in the 3.0.4 release notes before merging the PR.

Made-with: Cursor

---------

Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

Author: e-kwsm

docs/changelog.md

@@ -33,6 +33,9 @@ Internal:
 
 Bug fixes:
 
+- Fixed test builds with installed Eigen 5 by improving `Eigen3` CMake package detection.
+  [#6036](https://github.com/pybind/pybind11/pull/6036)
+
 - Fixed move semantics of `scoped_ostream_redirect` to preserve buffered output and avoid crashes when moved redirects restore stream buffers.
   [#6033](https://github.com/pybind/pybind11/pull/6033)
 

tests/CMakeLists.txt

@@ -300,10 +300,17 @@ if(PYBIND11_TEST_FILES_EIGEN_I GREATER -1)
 
   else()
     find_package(Eigen3 3.2.7 QUIET CONFIG)
+    if(NOT Eigen3_FOUND)
+      find_package(Eigen3 5 QUIET CONFIG)
+    endif()
+    set(EIGEN3_FOUND ${Eigen3_FOUND})
+    set(EIGEN3_VERSION ${Eigen3_VERSION})
 
     if(NOT EIGEN3_FOUND)
       # Couldn't load via target, so fall back to allowing module mode finding, which will pick up
       # tools/FindEigen3.cmake
+      # This MODULE-mode fallback is for older Eigen 3 setups; Eigen 5 is expected to be found
+      # via the CONFIG-mode probes above.
       find_package(Eigen3 3.2.7 QUIET)
     endif()
   endif()