require Kokkos 3.7 with device support

Author: tjhei

cmake/modules/FindDEAL_II_KOKKOS.cmake

@@ -124,13 +124,14 @@ if(KOKKOS_FOUND)
   set(KOKKOS_VERSION ${Kokkos_VERSION})
 
   if (Kokkos_ENABLE_CUDA OR Kokkos_ENABLE_HIP)
-    # In version older than 3.7.0, Kokkos::Array::operator[] is not constexpr,
-    # so we use std::array instead.
-    if (Kokkos_VERSION VERSION_LESS 3.7.0)
-      # We are using std::array in device code which calls the host-only function
-      # __glibcxx_requires_subscript when defining _GLIBCXX_ASSERTIONS 
-      list(REMOVE_ITEM DEAL_II_DEFINITIONS_DEBUG "_GLIBCXX_ASSERTIONS")
-    endif()
+    # In versions older than 3.7.0, we would need several workarounds if
+    # device support is enabled:
+    # - Kokkos::Array::operator[] is not constexpr
+    # - std::array uses __glibcxx_requires_subscript (host-only)
+    # - KOKKOS_IF_ON_DEVICE doesn't exist
+    #
+    # So, to simplify our work, require a more modern version:
+    message(ERROR "Device support (CUDA/HIP) requires Kokkos version 3.7 or newer!")
   endif()
 
   if(Kokkos_ENABLE_CUDA)

doc/readme.html

@@ -654,7 +654,10 @@ <h4>Optional interfaces to other software packages</h4>
                 <a href="https://github.com/kokkos/kokkos" target="_top">Kokkos</a> implements a programming model in
                 C++ for writing performance portable applications targeting all major HPC platforms.
                 To use a self compiled version, pass <code>-DKOKKOS_DIR=/path/to/kokkos</code> to the deal.II CMake
-                call. We currently require Kokkos version 3.4 or newer (when bundled in Trilinos) or 3.7 or newer when configured separately.<br/>
+                call. We currently require Kokkos version 3.4 or newer (when bundled in Trilinos) or 3.7 or newer
+		when configured separately. For device support we require 3.7 or newer (stand-alone or part of Trilinos
+		16 or newer).
+		<br/>
                 The compiler must be able to compile code for all enabled backends. In case CUDA is enabled in Kokkos
                 and nvcc should be used as device compiler,
                 it is recommended to use the nvcc_wrapper script that comes with Kokkos as C++ compiler. clang++ or