Correct options on Eigen::MappedSparseMatrix & adding MSVC C4127 supp…

…ression around Eigen includes. (#3352)

* Adding MSVC C4127 suppression around Eigen includes.

* For MSVC 2015 only: also adding the C4127 suppression to test_eigen.cpp

* Copying original change from PR #3343, with extra line breaks to not run past 99 columns (our desired but currently not enforced limit).

include/pybind11/eigen.h

@@ -17,9 +17,23 @@
 
 #include "numpy.h"
 
+// The C4127 suppression was introduced for Eigen 3.4.0. In theory we could
+// make it version specific, or even remove it later, but considering that
+// 1. C4127 is generally far more distracting than useful for modern template code, and
+// 2. we definitely want to ignore any MSVC warnings originating from Eigen code,
+// it is probably best to keep this around indefinitely.
+#if defined(_MSC_VER)
+#  pragma warning(push)
+#  pragma warning(disable: 4127) // C4127: conditional expression is constant
+#endif
+
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
 
+#if defined(_MSC_VER)
+#  pragma warning(pop)
+#endif
+
 // Eigen prior to 3.2.7 doesn't have proper move constructors--but worse, some classes get implicit
 // move constructors that break things.  We could detect this an explicitly copy, but an extra copy
 // of matrices seems highly undesirable.
@@ -559,7 +573,9 @@ struct type_caster<Type, enable_if_t<is_eigen_sparse<Type>::value>> {
         if (!values || !innerIndices || !outerIndices)
             return false;
 
-        value = Eigen::MappedSparseMatrix<Scalar, Type::Flags, StorageIndex>(
+        value = Eigen::MappedSparseMatrix<Scalar,
+                                          Type::Flags & (Eigen::RowMajor | Eigen::ColMajor),
+                                          StorageIndex>(
             shape[0].cast<Index>(), shape[1].cast<Index>(), nnz,
             outerIndices.mutable_data(), innerIndices.mutable_data(), values.mutable_data());
 

tests/CMakeLists.txt

@@ -174,9 +174,9 @@ set(PYBIND11_CROSS_MODULE_GIL_TESTS test_gil_scoped.py)
 set(PYBIND11_EIGEN_REPO
     "https://gitlab.com/libeigen/eigen.git"
     CACHE STRING "Eigen repository to use for tests")
-# This hash is for 3.3.8, using a hash for security reasons
+# This hash is for 3.4.0, using a hash for security reasons
 set(PYBIND11_EIGEN_VERSION
-    "dc252fbf00079ccab57948a164b1421703fe4361"
+    "929bc0e191d0927b1735b9a1ddc0e8b77e3a25ec"
     CACHE STRING "Eigen version to use for tests")
 
 # Check if Eigen is available; if not, remove from PYBIND11_TEST_FILES (but

tests/test_eigen.cpp

@@ -13,6 +13,9 @@
 #include <pybind11/stl.h>
 
 #if defined(_MSC_VER)
+#if _MSC_VER < 1910  // VS 2015's MSVC
+#  pragma warning(disable: 4127) // C4127: conditional expression is constant
+#endif
 #  pragma warning(disable: 4996) // C4996: std::unary_negation is deprecated
 #endif