EIGEN_HAS_MAJOR_VERSION_5 -> EIGEN_HAS_CXX17,

more dox

Author: EricaCMitchell

src/TiledArray/external/eigen.h

@@ -63,8 +63,10 @@ TILEDARRAY_PRAGMA_GCC(diagnostic pop)
 #endif
 #endif  // EIGEN_USE_LAPACKE || EIGEN_USE_LAPACKE_STRICT
 
-#if EIGEN_VERSION_AT_LEAST(3, 5, 0)
-#define EIGEN_HAS_MAJOR_VERSION_5 1
+// Eigen >= 5.x is last to support C++14, future releases require C++17
+// https://gitlab.com/libeigen/eigen/-/releases/5.0.0
+#if EIGEN_MAJOR_VERSION > 4
+#define EIGEN_HAS_CXX17 1
 #endif
 
 TILEDARRAY_PRAGMA_GCC(diagnostic pop)

src/TiledArray/fwd.h

@@ -29,6 +29,8 @@
 // #include <boost/container/detail/std_fwd.hpp>
 
 // fwddecl Eigen::aligned_allocator
+// N.B. with Eigen >= 5.x, aligned_allocator no longer inherits from
+// std::allocator https://gitlab.com/libeigen/eigen/-/merge_requests/1795
 namespace Eigen {
 template <class>
 class aligned_allocator;

src/TiledArray/tensor/tensor.h

@@ -185,8 +185,8 @@ class Tensor {
       : range_(range), nbatch_(nbatch) {
     size_t size = range_.volume() * nbatch;
     allocator_type allocator;
-#if EIGEN_HAS_MAJOR_VERSION_5
-    // Eigen >=3.5.0 returns nullptr if size=0
+#if EIGEN_HAS_CXX17
+    // Eigen (>= 5.x) allocate returns nullptr if size=0
     auto* ptr = (!size && range_.rank())
                     ? static_cast<pointer>(std::malloc(size))
                     : allocator.allocate(size);
@@ -233,8 +233,8 @@ class Tensor {
       : range_(std::move(range)), nbatch_(nbatch) {
     size_t size = range_.volume() * nbatch;
     allocator_type allocator;
-#if EIGEN_HAS_MAJOR_VERSION_5
-    // Eigen >=3.5.0 returns nullptr if size=0
+#if EIGEN_HAS_CXX17
+    // Eigen (>= 5.x) allocate returns nullptr if size=0
     auto* ptr = (!size && range_.rank())
                     ? static_cast<pointer>(std::malloc(size))
                     : allocator.allocate(size);