ARROW-5252: [C++] Use standard-compliant std::variant backport

Replace mapbox::variant with Michael Park's variant implementation.

Author: Antoine Pitrou <antoine@python.org>

Closes #4259 from pitrou/ARROW-5252-variant-backport and squashes the following commits:

03dbc0e <Antoine Pitrou> ARROW-5252:  Use standard-compliant std::variant backport

LICENSE.txt

@@ -406,37 +406,6 @@ DEALINGS IN THE SOFTWARE.
 
 --------------------------------------------------------------------------------
 
-This project includes code from the mapbox/variant project, BSD 3-clause
-license
-
-Copyright (c) MapBox
-All rights reserved.
-
-Redistribution and use in source and binary forms, with or without modification,
-are permitted provided that the following conditions are met:
-
-- Redistributions of source code must retain the above copyright notice, this
-  list of conditions and the following disclaimer.
-- Redistributions in binary form must reproduce the above copyright notice, this
-  list of conditions and the following disclaimer in the documentation and/or
-  other materials provided with the distribution.
-- Neither the name "MapBox" nor the names of its contributors may be
-  used to endorse or promote products derived from this software without
-  specific prior written permission.
-
-THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
-ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR
-ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
-(INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
-LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
-ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
-(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
-SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
---------------------------------------------------------------------------------
-
 This project includes code from the FlatBuffers project
 
 Copyright 2014 Google Inc.
@@ -709,6 +678,34 @@ DEALINGS IN THE SOFTWARE.
 
 --------------------------------------------------------------------------------
 
+The file cpp/src/arrow/vendored/variant.hpp has the following license
+
+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+
+--------------------------------------------------------------------------------
+
 The files in cpp/src/arrow/vendored/xxhash/ have the following license
 (BSD 2-Clause License)
 

c_glib/gandiva-glib/node.cpp

@@ -33,7 +33,7 @@ ggandiva_literal_node_get(GGandivaLiteralNode *node)
 {
   auto gandiva_literal_node =
     std::static_pointer_cast<gandiva::LiteralNode>(ggandiva_node_get_raw(GGANDIVA_NODE(node)));
-  return gandiva_literal_node->holder().get<Type>();
+  return arrow::util::get<Type>(gandiva_literal_node->holder());
 }
 
 G_BEGIN_DECLS

cpp/src/arrow/compute/kernel.h

@@ -117,6 +117,11 @@ struct ARROW_EXPORT Datum {
 
   Datum(const Datum& other) noexcept { this->value = other.value; }
 
+  Datum& operator=(const Datum& other) noexcept {
+    value = other.value;
+    return *this;
+  }
+
   // Define move constructor and move assignment, for better performance
   Datum(Datum&& other) noexcept : value(std::move(other.value)) {}
 
@@ -126,7 +131,7 @@ struct ARROW_EXPORT Datum {
   }
 
   Datum::type kind() const {
-    switch (this->value.which()) {
+    switch (this->value.index()) {
       case 0:
         return Datum::NONE;
       case 1:

cpp/src/arrow/scalar.h

@@ -30,7 +30,6 @@
 #include "arrow/type_fwd.h"
 #include "arrow/type_traits.h"
 #include "arrow/util/decimal.h"
-#include "arrow/util/variant.h"
 #include "arrow/util/visibility.h"
 
 namespace arrow {

cpp/src/arrow/util/variant.h

@@ -18,16 +18,16 @@
 #ifndef ARROW_UTIL_VARIANT_H
 #define ARROW_UTIL_VARIANT_H
 
-#include "arrow/vendored/variant/variant.hpp"  // IWYU pragma: export
-#include "arrow/vendored/variant/variant_io.hpp"
+#include "arrow/vendored/variant.hpp"  // IWYU pragma: export
 
 namespace arrow {
 namespace util {
 
-using mapbox::util::apply_visitor;  // seems akin to std::visit
-using mapbox::util::bad_variant_access;
-using mapbox::util::get;
-using mapbox::util::variant;
+using ::mpark::bad_variant_access;
+using ::mpark::get;
+using ::mpark::get_if;
+using ::mpark::variant;
+using ::mpark::visit;
 
 }  // namespace util
 }  // namespace arrow

cpp/src/arrow/vendored/CMakeLists.txt

@@ -18,4 +18,3 @@
 arrow_install_all_headers("arrow/vendored")
 
 add_subdirectory(datetime)
-add_subdirectory(variant)