nix: patch eigen

Nixpkgs uses eigen v3.4.0-unstable-2022-04-19,
aka https://gitlab.com/libeigen/eigen/-/merge_requests/974,
because that is required by onnxruntime.
Ref. NixOS/nixpkgs#364362

But that fix actually break other things, which are then fixed in
https://gitlab.com/libeigen/eigen/-/merge_requests/977

So we need to apply that here, and hope for the next eigen version

Author: nim65s

flake.nix

@@ -20,20 +20,29 @@
           devShells.default = pkgs.mkShell { inputsFrom = [ self'.packages.default ]; };
           packages = {
             default = self'.packages.eigenpy;
-            eigenpy = pkgs.python3Packages.eigenpy.overrideAttrs (_: {
-              src = pkgs.lib.fileset.toSource {
-                root = ./.;
-                fileset = pkgs.lib.fileset.unions [
-                  ./CMakeLists.txt
-                  ./doc
-                  ./include
-                  ./package.xml
-                  ./python
-                  ./src
-                  ./unittest
-                ];
-              };
-            });
+            eigen = pkgs.eigen.overrideAttrs {
+              # Apply https://gitlab.com/libeigen/eigen/-/merge_requests/977
+              postPatch = ''
+                substituteInPlace Eigen/src/SVD/BDCSVD.h \
+                  --replace-fail "if (l == 0) {" "if (i >= k && l == 0) {"
+              '';
+            };
+            eigenpy =
+              (pkgs.python3Packages.eigenpy.override { inherit (self'.packages) eigen; }).overrideAttrs
+                (_: {
+                  src = pkgs.lib.fileset.toSource {
+                    root = ./.;
+                    fileset = pkgs.lib.fileset.unions [
+                      ./CMakeLists.txt
+                      ./doc
+                      ./include
+                      ./package.xml
+                      ./python
+                      ./src
+                      ./unittest
+                    ];
+                  };
+                });
           };
         };
     };