Add fix for std::optional<Eigen::Ref<const T>>

bindings/python/src/expose-results.hpp

@@ -6,7 +6,7 @@
 #include <nanobind/eigen/dense.h>
 #include <nanobind/eigen/sparse.h>
 #include <nanobind/operators.h>
-#include <nanobind/stl/optional.h>
+#include "optional-eigen-fix.hpp"
 
 #include <proxsuite/helpers/optional.hpp>
 #include <proxsuite/serialization/archive.hpp>

bindings/python/src/expose-solve.hpp

@@ -6,7 +6,7 @@
 #include <nanobind/nanobind.h>
 #include <nanobind/eigen/dense.h>
 #include <nanobind/eigen/sparse.h>
-#include <nanobind/stl/optional.h>
+#include "optional-eigen-fix.hpp"
 
 namespace proxsuite {
 namespace proxqp {

bindings/python/src/optional-eigen-fix.hpp

@@ -0,0 +1,45 @@
+//
+// Copyright (c) 2024 INRIA
+//
+#pragma once
+
+#include <nanobind/stl/optional.h>
+#include <Eigen/Core>
+
+NAMESPACE_BEGIN(NB_NAMESPACE)
+NAMESPACE_BEGIN(detail)
+
+/// Fix std::optional for Eigen::Ref<const T>
+/// Credit to github.com/WKarel for this suggestion!
+/// https://github.com/wjakob/nanobind/issues/682#issuecomment-2310746145
+template<typename T>
+struct type_caster<std::optional<Eigen::Ref<const T>>>
+  : optional_caster<std::optional<Eigen::Ref<const T>>>
+{
+  using Ref = Eigen::Ref<const T>;
+  using Optional = std::optional<Ref>;
+  using Caster = typename type_caster::optional_caster::Caster;
+  using Map = typename Caster::Map;
+  using DMap = typename Caster::DMap;
+  NB_TYPE_CASTER(Optional, optional_name(Caster::Name))
+
+  bool from_python(handle src, uint8_t flags, cleanup_list* cleanup) noexcept
+  {
+    if (src.is_none())
+      return true;
+    Caster caster;
+    if (!caster.from_python(src, flags_for_local_caster<T>(flags), cleanup) ||
+        !caster.template can_cast<T>())
+      return false;
+    /// This allows us to bypass the type_caster for Eigen::Ref
+    /// which is broken due to lack of NRVO + move ctor in latest Eigen release.
+    if (caster.dcaster.caster.value.is_valid())
+      value.emplace(caster.dcaster.operator DMap());
+    else
+      value.emplace(caster.caster.operator Map());
+    return true;
+  }
+};
+
+NAMESPACE_END(detail)
+NAMESPACE_END(NB_NAMESPACE)