diff -rupN --no-dereference eigen-3.3.9/Eigen/src/Core/MathFunctions.h eigen-3.3.9-new/Eigen/src/Core/MathFunctions.h

--- eigen-3.3.9/Eigen/src/Core/MathFunctions.h	2020-12-04 22:53:41.000000000 +0100

+++ eigen-3.3.9-new/Eigen/src/Core/MathFunctions.h	2021-01-19 12:32:05.328624222 +0100

@@ -485,12 +485,22 @@ struct log1p_impl {

     EIGEN_STATIC_ASSERT_NON_INTEGER(Scalar)

     #if EIGEN_HAS_CXX11_MATH

     using std::log1p;

-    #endif

+    #else

     using std_fallback::log1p;

+    #endif

     return log1p(x);

   }

 };

+// Specialization for complex types that are not supported by std::log1p.

+template <typename RealScalar>

+struct log1p_impl<std::complex<RealScalar> > {

+  EIGEN_DEVICE_FUNC static inline std::complex<RealScalar> run(

+      const std::complex<RealScalar>& x) {

+    EIGEN_STATIC_ASSERT_NON_INTEGER(RealScalar)

+    return std_fallback::log1p(x);

+  }

+};

 template<typename Scalar>

 struct log1p_retval