Replace type aliases deprecated/removed in C++17/C++20 from FixedArray

- Raised as issue #551
- Also adds C++20 to the set of ALLOWED_CXX_STANDARDS, although this
  will require a version of CMake >= 3.12.

Change-Id: I0f13c72e93a35391fd2d18590b4243a329a2322c

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7de7d37..5a0de03 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -44,7 +44,7 @@
   "C++ standard (minimum 11)" FORCE)
 # Restrict CMAKE_CXX_STANDARD to the valid versions permitted and ensure that
 # if one was forced via -D that it is in the valid set.
-set(ALLOWED_CXX_STANDARDS 11 14 17)
+set(ALLOWED_CXX_STANDARDS 11 14 17 20)
 set_property(CACHE CMAKE_CXX_STANDARD PROPERTY STRINGS ${ALLOWED_CXX_STANDARDS})
 list(FIND ALLOWED_CXX_STANDARDS ${CMAKE_CXX_STANDARD} POSITION)
 if (POSITION LESS 0)

include/ceres/internal/fixed_array.h

diff --git a/include/ceres/internal/fixed_array.h b/include/ceres/internal/fixed_array.h
index 15481f3..021851d 100644
--- a/include/ceres/internal/fixed_array.h
+++ b/include/ceres/internal/fixed_array.h
@@ -105,13 +105,13 @@
 
  public:
   using allocator_type = typename AllocatorTraits::allocator_type;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;