Add std::variant backport as c10::variant (pytorch#26836)

Summary:
Pull Request resolved: pytorch#26836

* **pytorch#26836 Add std::variant backport as c10::variant**

Test Plan: Imported from OSS

Differential Revision: D17649064

Pulled By: yf225

fbshipit-source-id: aa5ee26fe7078cc66d03663b9ff9e998e1d5839a

Author: Will Feng

aten/src/ATen/test/CMakeLists.txt

@@ -29,7 +29,8 @@ list(APPEND ATen_CPU_TEST_SRCS
   ${CMAKE_CURRENT_SOURCE_DIR}/xla_tensor_test.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/tensor_iterator_test.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/cpu_generator_test.cpp
-  ${CMAKE_CURRENT_SOURCE_DIR}/pow_test.cpp)
+  ${CMAKE_CURRENT_SOURCE_DIR}/pow_test.cpp
+  ${CMAKE_CURRENT_SOURCE_DIR}/variant_test.cpp)
 
 list(APPEND ATen_CUDA_TEST_SRCS
   ${CMAKE_CURRENT_SOURCE_DIR}/cuda_integer_divider_test.cu

aten/src/ATen/test/variant_test.cpp

@@ -0,0 +1,41 @@
+#include <gtest/gtest.h>
+
+#include <c10/util/variant.h>
+
+namespace testns {
+
+namespace enumtype {
+  // NOTE: We need to provide the default constructor for each struct,
+  // otherwise Clang 3.8 would complain:
+  // ```
+  // error: default initialization of an object of const type 'const enumtype::Enum1'
+  // without a user-provided default constructor
+  // ```
+  struct Enum1 { Enum1() {} };
+  struct Enum2 { Enum2() {} };
+  struct Enum3 { Enum3() {} };
+} // namespace enumtype
+
+const enumtype::Enum1 kEnum1;
+const enumtype::Enum2 kEnum2;
+const enumtype::Enum3 kEnum3;
+
+} // namespace testns
+
+std::string func(c10::variant<testns::enumtype::Enum1, testns::enumtype::Enum2, testns::enumtype::Enum3> v) {
+  if (c10::get_if<testns::enumtype::Enum1>(&v)) {
+    return "Enum1";
+  } else if (c10::get_if<testns::enumtype::Enum2>(&v)) {
+    return "Enum2";
+  } else if (c10::get_if<testns::enumtype::Enum3>(&v)) {
+    return "Enum3";
+  } else {
+    return "Unsupported enum";
+  }
+}
+
+TEST(VariantTest, Basic) {
+  ASSERT_EQ(func(testns::kEnum1), "Enum1");
+  ASSERT_EQ(func(testns::kEnum2), "Enum2");
+  ASSERT_EQ(func(testns::kEnum3), "Enum3");
+}