Change array_size result from enum to constexpr.

Eigen/src/Core/util/EmulateArray.h

diff --git a/Eigen/src/Core/util/EmulateArray.h b/Eigen/src/Core/util/EmulateArray.h
index 2b11552..4bfbbba 100644
--- a/Eigen/src/Core/util/EmulateArray.h
+++ b/Eigen/src/Core/util/EmulateArray.h
@@ -204,19 +204,19 @@
 
 template <class T, std::size_t N>
 struct array_size<array<T, N> > {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 template <class T, std::size_t N>
 struct array_size<array<T, N>&> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 template <class T, std::size_t N>
 struct array_size<const array<T, N> > {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 template <class T, std::size_t N>
 struct array_size<const array<T, N>&> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 
 }  // end namespace internal

Eigen/src/Core/util/Meta.h

diff --git a/Eigen/src/Core/util/Meta.h b/Eigen/src/Core/util/Meta.h
index 859d2f1..d2336ce 100644
--- a/Eigen/src/Core/util/Meta.h
+++ b/Eigen/src/Core/util/Meta.h
@@ -303,30 +303,30 @@
  */
 template <typename T, typename EnableIf = void>
 struct array_size {
-  enum { value = Dynamic };
+  static constexpr Index value = Dynamic;
 };
 
 template <typename T>
 struct array_size<T, std::enable_if_t<((T::SizeAtCompileTime & 0) == 0)>> {
-  enum { value = T::SizeAtCompileTime };
+  static constexpr Index value = T::SizeAtCompileTime;
 };
 
 template <typename T, int N>
 struct array_size<const T (&)[N]> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 template <typename T, int N>
 struct array_size<T (&)[N]> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 
 template <typename T, std::size_t N>
 struct array_size<const std::array<T, N>> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 template <typename T, std::size_t N>
 struct array_size<std::array<T, N>> {
-  enum { value = N };
+  static constexpr Index value = N;
 };
 
 /** \internal